Form 424B3 COLUMBIA BANKING SYSTEM,

Filed Pursuant to Rule 424(b)(3)
Registration Statement No. 333-258551

MERGER PROPOSED—YOUR VOTE IS VERY IMPORTANT

Dear Shareholders of Bank of Commerce Holdings,

As previously announced, Bank of Commerce Holdings, which we refer to as BOCH, and Columbia Banking System, Inc., which we refer to as Columbia, have entered into a definitive merger agreement, which we refer to as the merger agreement, that provides for the combination of the two companies. Under the merger agreement, BOCH will merge with and into Columbia, with Columbia as the surviving entity, which we refer to as the parent merger. Promptly following the parent merger, Merchants Bank of Commerce, a California state-chartered bank and wholly owned subsidiary of BOCH, which we refer to as Merchants Bank, will merge with and into Columbia State Bank, a Washington state-chartered bank and wholly owned subsidiary of Columbia, which we refer to as Columbia Bank, with Columbia Bank as the surviving bank, which we refer to as the bank merger. We refer to the parent merger and the bank merger collectively as the mergers. We cannot complete the mergers until the shareholders of BOCH approve the principal terms of the merger agreement. BOCH shareholders will be asked to vote on whether to approve the principal terms of the merger agreement at the special meeting of shareholders to be held on September 21, 2021.

Under the terms of the merger agreement, BOCH shareholders will have the right to receive 0.40 of a share of Columbia common stock with respect to each of their shares of BOCH common stock (subject to any adjustment as provided in the merger agreement), which we refer to as the merger consideration. As of June 23, 2021, the date the mergers were announced, based on the expected issuance of 6,758,313 Columbia common shares in the parent merger, the total merger consideration payable to holders of BOCH common shares was valued at approximately $266.0 million. As of the date of this proxy statement/prospectus, based on the expected issuance of 6,757,479 Columbia common shares in the parent merger, the total merger consideration payable to holders of BOCH common shares is valued at approximately $251.0 million. We expect the transactions to be tax-free for BOCH shareholders. After completion of the parent merger, based on the current issued and outstanding Columbia common shares and the 6,757,479 Columbia common shares expected to be issued to BOCH shareholders, BOCH shareholders would own approximately 8.6% of Columbia’s common shares (ignoring any shares of Columbia common shares they may already own).

Subject to the terms and conditions of the merger agreement, at the date and time when the parent merger becomes effective, which we refer to as the effective time, each share of BOCH’s common stock issued and outstanding will be converted into the right to receive from Columbia 0.40 of a share of Columbia common stock, which we refer to as the exchange ratio. The exchange ratio is subject to adjustment to the extent that specified expenses incurred by BOCH in connection with the transaction total in excess of $10,500,000 ($10,750,000 if the effective time occurs in 2022) and in certain other circumstances as described below.

BOCH may elect to terminate the merger agreement if (a) the Columbia average closing price is less than $35.48 per share and (b) the percentage decline in the Columbia average closing price underperforms the KBW Regional Banking Index over the same time period by greater than 15%, provided that BOCH has given notice of its election to exercise its right to terminate the merger agreement within one business day following the determination date and Columbia does not, within two business days following such notice, agree to adjust the exchange ratio so that the termination above does not occur.

You should obtain current price quotations for Columbia common shares and BOCH common shares. Columbia common shares are traded on the Nasdaq Global Select Market under the symbol “COLB,” and BOCH common shares are traded on the Nasdaq Global Market under the symbol “BOCH.”

BOCH’s board of directors has unanimously determined that the combination of BOCH and Columbia is in the best interests of BOCH and its shareholders based upon its analysis, investigation and deliberation, and the BOCH board of directors unanimously recommends that the BOCH shareholders vote “FOR” the approval of the principal terms of the merger agreement and “FOR” the approval of the other proposals described in this proxy statement/prospectus.

You should read this entire proxy statement/prospectus carefully, including the appendices and the documents incorporated by reference into the document, because it contains important information about the mergers and the related transactions. In particular, you should read carefully the information under the section entitled “Risk Factors” beginning on page 15 for a discussion of the risks you should consider in evaluating the proposed mergers and how they will affect you.

The Columbia common shares to be issued to BOCH shareholders in the parent merger are not deposits or savings accounts or other obligations of any bank or savings association, and are not insured by the Federal Deposit Insurance Corporation or any other governmental agency.

Sincerely,

Randall S. Eslick

President and Chief Executive Officer

Bank of Commerce Holdings

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved the mergers described in this proxy statement/prospectus or the Columbia common shares to be issued in the parent merger, or passed upon the adequacy or accuracy of this proxy statement/prospectus. Any representation to the contrary is a criminal offense.

This proxy statement/prospectus is dated August 17, 2021, and is first being mailed to the shareholders of BOCH on or about August 18, 2021.

Bank of Commerce Holdings

555 Capitol Mall, Suite 1255

Sacramento, California 95814

NOTICE OF SPECIAL MEETING OF SHAREHOLDERS

TO BE HELD ON SEPTEMBER 21, 2021

To the Shareholders of Bank of Commerce Holdings:

Bank of Commerce Holdings, which we refer to as BOCH, will hold a special meeting of shareholders at 1015 7th Street, Sacramento, California 95814, at 4:00 p.m., Pacific Time, on September 21, 2021, which we refer to as the BOCH special meeting, to consider and vote upon the following matters:

BOCH will transact no other business at the BOCH special meeting, except for business properly brought before the BOCH special meeting or any adjournment or postponement of such meeting.

The approval by BOCH’s shareholders of the merger proposal is required for the completion of the merger described in this proxy statement/prospectus.

We have fixed the close of business on August 11, 2021, as the record date for the BOCH special meeting. Only BOCH common shareholders of record at that time are entitled to notice of, and to vote at, the BOCH special meeting, or any adjournment or postponement of the BOCH special meeting. Approval of the merger proposal requires the affirmative vote of the holders of a majority of all outstanding BOCH common shares entitled to vote. Approval of the merger-related named executive officer compensation proposal and the adjournment proposal requires the affirmative vote of a majority of the BOCH common shares represented and voting.

Your vote is very important. To ensure your representation at the BOCH special meeting, please submit your proxy through the Internet or by telephone or complete and return the enclosed proxy card. Please vote promptly whether or not you expect to attend the BOCH special meeting. Submitting a proxy now will not prevent you from being able to vote in person at the BOCH special meeting.

The BOCH board of directors has adopted and approved the merger agreement and the transactions contemplated thereby, and unanimously recommends that you vote “FOR” each of the proposals.

By order of the Board of Directors,

Andrea M. Newburn

Corporate Secretary

Sacramento, California

August 17, 2021

WHERE YOU CAN FIND MORE INFORMATION

Both Columbia Banking System, Inc., which we refer to as Columbia, and Bank of Commerce Holdings, which we refer to as BOCH, file annual, quarterly and special reports, proxy statements and other business and financial information with the Securities and Exchange Commission, which we refer to as the SEC. You may read the materials that either Columbia or BOCH files with the SEC at the SEC’s website located at http://www.sec.gov. You can also obtain, free of charge, documents that Columbia files with the SEC at www.columbiabank.com under the tab “About Us” and then under the heading “Investor Relations” or documents that BOCH files with the SEC at www.bankofcommerceholdings.com under the tab “Investor Services” and then under the heading “Corporate Profile.” The information provided on the Columbia and BOCH websites is not part of this proxy statement/prospectus and is not incorporated herein by reference. Copies of the documents that Columbia or BOCH, respectively, files with the SEC can also be obtained, free of charge, by directing a written request to Columbia Banking System, Inc., Attention: Corporate Secretary, 1301 “A” Street, Suite 800, Tacoma, Washington 98402 or to Bank of Commerce Holdings, Attention: Corporate Secretary, 555 Capitol Mall, Suite 1255, Sacramento, California 95814.

Columbia has filed a registration statement on Form S-4 to register with the SEC Columbia common shares as specified therein. This proxy statement/prospectus is a part of that registration statement. As permitted by SEC rules, this document does not contain all of the information included in the registration statement or in the exhibits or schedules to the registration statement. You may read and copy the registration statement, including any amendments, schedules and exhibits, at the addresses set forth below. Statements contained in this document as to the contents of any contract or other documents referred to in this document are not necessarily complete. In each case, you should refer to the copy of the applicable contract or other document filed as an exhibit to the registration statement. This document incorporates important business and financial information about Columbia and BOCH that is not included in or delivered with this document, including incorporating by reference documents that Columbia and BOCH have previously filed with the SEC. These documents contain important information about the companies and their financial condition. See “Documents Incorporated by Reference.” These documents are available without charge to you upon written or oral request to the applicable company’s principal executive offices. The respective addresses and telephone numbers of such principal executive offices are listed below:

To obtain timely delivery of these documents, you must request the information no later than September 13, 2021, in order to receive them before the special meeting of BOCH shareholders, which we refer to as the BOCH special meeting.

Columbia common shares, no par value per share, which we refer to as Columbia common shares, are traded on the Nasdaq Global Select Market under the symbol “COLB,” and BOCH common shares, no par value, which we refer to as BOCH common shares, are traded on the Nasdaq Global Market under the symbol “BOCH.”

Table of Contents

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QUESTIONS AND ANSWERS

The following questions and answers briefly address some commonly asked questions about the transactions and the shareholder meeting. They may not include all the information that is important to the shareholders of BOCH. Shareholders of BOCH should carefully read this entire proxy statement/prospectus, including the appendices and other documents referred to in this document.

The parent merger cannot be completed unless BOCH shareholders approve the principal terms of the merger agreement. BOCH is holding a special meeting of shareholders to vote on the proposals necessary to complete the parent merger. Information about the special meeting and the mergers is contained in this proxy statement/prospectus.

This document constitutes both a proxy statement of BOCH and a prospectus of Columbia. It is a proxy statement because the BOCH board of directors is soliciting proxies from its shareholders. It is a prospectus because Columbia will issue Columbia common shares in exchange for BOCH common shares in the parent merger.

If immediately prior to the effective time, the transaction expenses incurred by BOCH exceed $10.5 million, or $10.75 million if the closing date is in calendar year 2022, which we refer to as the threshold amount, there will be an adjustment made to the exchange ratio such that the excess amount of transaction expenses will be deducted from the aggregate merger consideration. In that event, the exchange ratio will be equal to the product of (x) the exchange ratio and (y) the quotient obtained by dividing (1) the aggregate merger consideration, as defined below, minus the reduction amount, as defined below, by (2) the aggregate merger consideration. The aggregate merger consideration will equal (A) the Columbia average closing price multiplied by 0.40, multiplied by (B) the total number of BOCH common shares issued and outstanding immediately prior to the closing (other than treasury shares). The reduction amount means, to the extent the

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transaction expenses exceed the threshold amount, (a) the transaction expenses, minus (b) the threshold amount, plus (c) any tax benefits estimated to result from the transaction expenses exceeding the threshold amount, as reasonably estimated by Columbia. In the event of a transaction or series of related transactions in which a person acquires record or beneficial ownership of Columbia common shares that, together with shares held by such person, constitutes greater than 50% of the outstanding Columbia common shares, the reduction amount will be zero.

In addition, if (a) the Columbia average closing price is less than $35.48, which we refer to as the signing price, and (b) the Columbia average closing price underperforms the KBW Regional Banking Index by greater than 15%, then BOCH can terminate the merger agreement unless Columbia, at its option and in its sole discretion, adjusts the exchange ratio to a number equal to the lesser of (A) a quotient, the numerator of which is the signing price multiplied by the exchange ratio, and the denominator of which is the Columbia average closing price, or (B) the product of (1) a quotient, the numerator of which is the signing price multiplied by the exchange ratio, and the denominator of which is the Columbia average closing price, multiplied by (2) the percent change in the KBW Regional Banking Index.

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Please follow the instructions set forth on the proxy card or on the voting instruction form provided by the record holder if your shares are held in the name of your broker, bank or other nominee.

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You may also cast your vote in person at the BOCH special meeting.

If your shares are held in “street name” through a broker, bank or other nominee, that institution will send you separate instructions describing the procedure for voting your shares. Holders in “street name” who wish to vote in person at the BOCH special meeting will need to obtain a proxy form from the institution that holds their shares.

Brokers, banks or other nominees who hold shares in street name for a beneficial owner typically have the authority to vote in their discretion on “routine” proposals when they have not received instructions from beneficial owners. However, brokers, banks or other nominees are not allowed to exercise their voting discretion on matters that are determined to be “non-routine” without specific instructions from the beneficial owner. Broker non-votes are shares held by a broker, bank or other nominee that are represented at the shareholder meeting but with respect to which the broker, bank or other nominee is not instructed by the beneficial owner of such shares to vote on the particular proposal and the broker, bank or other nominee does not have discretionary voting power on such proposal.

If you are a BOCH shareholder and you do not instruct your broker, bank or other nominee on how to vote your shares, your broker, bank or other nominee may not vote your shares on the merger proposal, which broker non-votes will have the same effect as a vote “AGAINST” the proposal. If you do not instruct your broker, bank or other nominee on how to vote your shares, your broker, bank or other nominee may not vote your shares on the merger-related named executive compensation proposal or BOCH adjournment proposal, which broker non-votes will have no effect on the vote on these proposals.

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The Merger-Related Named Executive Officer Compensation Proposal. The affirmative vote of a majority of the BOCH common shares represented and voting on the merger-related named executive officer compensation proposal is required to approve the proposal.

The BOCH Adjournment Proposal. The affirmative vote of a majority of the BOCH common shares represented and voting on the BOCH adjournment proposal is required to approve the proposal.

For purposes of the merger proposal, if a BOCH shareholder present in person at the BOCH special meeting abstains from voting or responds by proxy with an “abstain” vote, it will have the same effect as a vote cast “AGAINST” the merger proposal. If a BOCH shareholder is not present in person at the BOCH special meeting and does not respond by proxy, it also will have the same effect as a vote cast “AGAINST” the merger proposal.

For purposes of the merger-related named executive officer compensation proposal and the BOCH adjournment proposal, if a BOCH shareholder is present in person at the BOCH special meeting and abstains from voting or responds by proxy with an “abstain” vote, or is not present in person at the BOCH special meeting and does not respond by proxy, it will have no effect on the vote on these proposals.

If you choose any of the first three methods, you must take the described action (and, in the case of the second method, your proxy card must be received) no later than 11:59 p.m. Eastern Time on the day prior to the special meeting, unless you are voting shares held in the 401(k) Plan, in which case the deadline is 11:59 p.m. Eastern Time on September 13, 2021.

If your shares are held in an account at a broker, bank or other nominee, you should contact your broker, bank or other nominee to change your vote.

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Accordingly, based on the opinions to be delivered in connection with the completion of the parent merger, you generally will not recognize any gain or loss upon receipt of Columbia common shares in exchange for BOCH common shares in the parent merger (except with respect to any cash received in lieu of a fractional Columbia common share, as discussed below under “Material U.S. Federal Income Tax Consequences of the Parent Merger—Cash Received In Lieu of a Fractional Columbia Common Share”).

For a more detailed discussion of the material U.S. federal income tax consequences of the parent merger, please see the section entitled “Material U.S. Federal Income Tax Consequences of the Parent Merger” beginning on page 59.

The tax consequences of the parent merger to any particular shareholder will depend on that shareholder’s particular facts and circumstances. Accordingly, you are urged to consult your tax advisor to determine your tax consequences from the mergers.

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RISK FACTORS

In addition to the other information contained in or incorporated by reference into this document, including Columbia’s Annual Report on Form 10-K for the fiscal year ended December 31, 2020 and Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2021, and BOCH’s Annual Report on Form 10-K for the fiscal year ended December 31, 2020 and Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2021, and the matters addressed under the caption “Cautionary Note Regarding Forward-Looking Statements,” BOCH shareholders should consider the matters described below carefully in determining whether to approve the principal terms of the merger agreement and the other proposals described herein.

Risk Factors Relating to the Mergers

Because the market price of Columbia common shares may fluctuate, you cannot be sure of the value of the merger consideration that you will receive.

Upon completion of the parent merger, each BOCH common share (other than treasury shares) will be converted into the right to receive merger consideration consisting of a number of Columbia common shares equal to the exchange ratio, pursuant to the terms of the merger agreement. Other than in certain circumstances described below, the number of Columbia common shares to be received by a BOCH shareholder will be determined based on a fixed exchange ratio of 0.40 of a Columbia common share for each BOCH common share (other than treasury shares). Accordingly, the value of the merger consideration to be received by the BOCH shareholders will be based on the value of the Columbia common shares at closing. The value of the Columbia common shares to be received by BOCH shareholders in the parent merger may vary from the value as of the date we announced the mergers, the date that this document was mailed to BOCH shareholders, the date of the BOCH special meeting and the determination period. Any change in the market price of Columbia common shares prior to completion of the parent merger will affect the value of the merger consideration that BOCH shareholders will receive upon completion of the parent merger. Accordingly, at the time of the BOCH special meeting, BOCH shareholders will not know or be able to calculate the value of the per share consideration they would receive upon completion of the parent merger. Share price changes may result from a variety of factors, including general market and economic conditions, changes in our respective businesses, operations and prospects, and regulatory considerations, among other things. Many of these factors are beyond the control of Columbia and BOCH. BOCH shareholders should obtain current market quotations for Columbia common shares before voting their shares at the BOCH special meeting.

In addition, the exchange ratio adjustment mechanisms provide that, in the event (a) the Columbia average closing price is less than the signing price and (b) the Columbia average closing price underperforms the KBW Regional Banking Index by greater than 15%, then BOCH can terminate the merger agreement unless Columbia, at its option and in its sole discretion, adjusts the exchange ratio to a number equal to the lesser of (A) a quotient, the numerator of which is the signing price multiplied by the exchange ratio, and the denominator of which is the Columbia average closing price, or (B) the product of (1) a quotient, the numerator of which is the signing price multiplied by the exchange ratio, and the denominator of which is the Columbia average closing price, multiplied by (2) the percent change in the KBW Regional Banking Index. Furthermore, if immediately prior to the effective time, the transaction expenses incurred by BOCH exceed $10.5 million, or $10.75 million if the closing date is in calendar year 2022, there will be an adjustment made to the exchange ratio such that the excess amount of transaction expenses will be deducted from the aggregate merger consideration. However, as noted above, at the time of the BOCH special meeting, BOCH shareholders may not know or be able to calculate the exchange ratio used to determine the number of Columbia common shares they would receive with respect to each BOCH common share upon the completion of the merger.

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The results of operations of Columbia after the mergers may be affected by factors different from those currently affecting the results of operations of BOCH.

The businesses of Columbia and BOCH differ in certain respects and, accordingly, the results of operations of the combined company and the market price of the combined company’s common shares may be affected by factors different from those currently affecting the independent results of operations of BOCH and Columbia. For a discussion of the business of Columbia and certain factors to be considered in connection with Columbia’s business, see “Information Concerning Columbia” and the documents incorporated by reference in this document and referred to under “Where You Can Find More Information.” For a discussion of the business of BOCH and certain factors to be considered in connection with BOCH’s business, see “Information Concerning BOCH” and the documents incorporated by reference in this document and referred to under “Where You Can Find More Information.”

The merger agreement limits BOCH’s ability to pursue an alternative transaction and requires BOCH to pay a termination fee of $12,000,000 under certain circumstances relating to alternative acquisition proposals.

The merger agreement prohibits BOCH from soliciting, initiating, encouraging or knowingly facilitating certain alternative acquisition proposals with any third party, subject to exceptions set forth in the merger agreement. See “The Merger Agreement—Covenants and Agreements—No Solicitation; Change of Board Recommendation” included elsewhere in this proxy statement/prospectus. The merger agreement also provides for the payment by BOCH to Columbia of a termination fee of $12,000,000 in the event that the merger agreement is terminated in certain circumstances involving, among others, the termination of the merger agreement in certain circumstances followed by an acquisition, or a definitive agreement providing for an acquisition, of BOCH by a third party. These provisions may discourage a potential competing acquirer that might have an interest in acquiring BOCH from considering or proposing such an acquisition. It should be noted, however, that the failure of BOCH shareholders to approve the principal terms of the merger agreement will not in and of itself trigger BOCH’s obligation to pay the termination fee, unless other factors, including a third-party acquisition proposal for BOCH, also exist. See “The Merger Agreement—Termination; Termination Fee” included elsewhere in this proxy statement/prospectus.

The opinion of Raymond James delivered to BOCH’s board of directors prior to the signing of the merger agreement does not reflect any changes in circumstance since the date of the opinion.

The BOCH board of directors received an opinion from Raymond James & Associates, Inc., which we refer to as Raymond James, on June 23, 2021, to address the fairness of the exchange ratio for BOCH shareholders from a financial point of view as of that date. Changes in the operations and prospects of Columbia or BOCH, general market and economic conditions and other factors that may be beyond the control of Columbia or BOCH may have altered the value of Columbia or BOCH or the market prices of shares of Columbia or BOCH as of the date of this proxy statement/prospectus, or may alter such values and market prices by the time the mergers are completed. Raymond James does not have any obligation to update, revise or reaffirm its opinion to reflect subsequent developments, and has not done so. The opinion did not address the fairness of the exchange ratio for BOCH shareholders from a financial point of view at the time the mergers are completed or as of any date other than the date of the opinion. However, BOCH’s board of directors’ unanimous recommendation that BOCH shareholders vote “FOR” approval of the principal terms of the merger agreement is made as of the date of this proxy statement/prospectus. For a description of the opinion that BOCH received from Raymond James, see “The Mergers—Opinion of Raymond James” included elsewhere in this proxy statement/prospectus.

The mergers are subject to the receipt of approvals from governmental entities that may impose conditions that could have an adverse effect on the combined company following the mergers.

Before the mergers may be completed, various approvals must be obtained from the Federal Reserve, the FDIC and the WDFI. These governmental entities may impose conditions on the granting of such approvals or require changes to the terms of the mergers. Although Columbia and BOCH do not currently expect that any such

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material conditions or changes would be imposed, there can be no assurance that they will not be, and such conditions or changes could have the effect of delaying or preventing completion of the mergers or imposing additional costs or limiting the revenues of the combined company following the mergers, any of which might have an adverse effect on the combined company following the mergers.

The parent merger is subject to certain closing conditions that, if not satisfied or waived, will result in the mergers not being completed, which may cause the prices of Columbia common shares or BOCH common shares to decline.

The parent merger is subject to customary conditions to closing, including the receipt of required regulatory approvals and approval of BOCH’s shareholders of certain merger-related proposals. If any condition to the parent merger is not satisfied or waived, to the extent permitted by law, the mergers will not be completed. In addition, Columbia and BOCH may terminate the merger agreement under certain circumstances even if the merger agreement is approved by BOCH’s shareholders. If Columbia and BOCH do not complete the parent merger, the trading prices of Columbia common shares or BOCH common shares may decline. In addition, neither company would realize any of the expected benefits of having completed the mergers. If the mergers are not completed and BOCH’s board of directors seeks another merger or business combination, BOCH shareholders cannot be certain that BOCH will be able to find a party willing to offer equivalent or more attractive consideration than the consideration Columbia has agreed to provide. If the mergers are not completed, additional risks could materialize, which could materially and adversely affect the business, financial condition and results of Columbia or BOCH. For more information on closing conditions to the merger agreement, see “The Merger Agreement—Conditions to the Parent Merger” included elsewhere in this proxy statement/prospectus.

Columbia and BOCH will be subject to business uncertainties and contractual restrictions while the mergers are pending.

Uncertainty about the effect of the mergers on employees, customers and vendors may have an adverse influence on the business, financial condition and results of operations of Columbia and BOCH. These uncertainties may impair Columbia’s or BOCH’s ability to attract, retain and motivate key personnel, depositors and borrowers pending the consummation of the mergers, as such personnel, depositors and borrowers may experience uncertainty about their future roles following the consummation of the mergers. Additionally, these uncertainties could cause customers (including depositors and borrowers), suppliers, vendors and others who deal with Columbia or BOCH to seek to change existing business relationships with Columbia, BOCH or the combined company or fail to extend an existing relationship with Columbia, BOCH or the combined company.

In addition, the merger agreement restricts BOCH from taking certain actions without Columbia’s consent while the mergers are pending. These restrictions could have a material adverse effect on BOCH’s business, financial condition and results of operations. Please see the section entitled “The Merger Agreement—Covenants and Agreements” for a description of the restrictive covenants applicable to BOCH.

Columbia common shares to be received by BOCH shareholders as a result of the parent merger will have rights different from the BOCH common shares.

Upon completion of the parent merger, the rights of former BOCH shareholders who receive Columbia common shares in the parent merger and thereby become Columbia shareholders will be governed by the Columbia articles and the Columbia bylaws. The rights associated with BOCH common shares are different from the rights associated with Columbia common shares. In addition, the rights of shareholders under Washington law, where Columbia is organized, may differ from the rights of shareholders under California law, where BOCH is organized. See “Comparison of Certain Rights of Holders of Columbia and BOCH Common Shares” for a discussion of the different rights associated with Columbia common shares.

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Columbia has various provisions in the Columbia articles that could impede a takeover of Columbia.

The Columbia articles contain provisions providing for, among other things, preferred stock, super majority approval of certain business transactions, and consideration of non-monetary factors in evaluating a takeover offer. Although these provisions were not adopted for the express purpose of preventing or impeding the takeover of Columbia without the approval of the Columbia board of directors, such provisions may have that effect. Such provisions may prevent former BOCH shareholders who receive shares of Columbia common shares in the parent merger from taking part in a transaction in which such shareholders could realize a premium over the current market price of Columbia common shares. See “Comparison of Certain Rights of Holders of Columbia and BOCH Common Shares.”

Columbia and BOCH shareholders will generally have a reduced ownership and voting interest after the parent merger and will exercise less influence over management.

Columbia shareholders currently have the right to vote in the election of the Columbia board of directors and on other matters affecting Columbia. BOCH shareholders currently have the right to vote in the election of the BOCH board of directors and on other matters affecting BOCH. Upon the completion of the parent merger, except for shareholders who own common shares in both Columbia and BOCH, each party’s shareholders will be a shareholder of Columbia with a percentage ownership of Columbia that is smaller than such shareholder’s current percentage ownership of Columbia or BOCH, as applicable. It is currently expected that the former shareholders of BOCH as a group will receive shares in the parent merger constituting approximately 9% of the outstanding shares of the combined company immediately after the parent merger. As a result, current shareholders of Columbia as a group will own approximately 91% of the outstanding shares of the combined company immediately after the parent merger. Because of this, Columbia and BOCH shareholders will generally have less influence on the management and policies of the combined company than they now have on the management and policies of Columbia or BOCH, as applicable.

Sales of substantial amounts of Columbia’s common shares in the open market by former BOCH shareholders could depress Columbia’s share price.

Columbia common shares that are issued to shareholders of BOCH in the parent merger will be freely tradable without restrictions or further registration under the Securities Act of 1933, as amended, which we refer to as the Securities Act. As of June 21, 2021, Columbia had approximately 71,743,885 common shares outstanding and 2,210,216 Columbia common shares were reserved for issuance under the Columbia stock plans. Based on the number of BOCH common shares outstanding as of the date of this proxy statement/prospectus, Columbia expects to issue approximately 6,757,479 common shares in connection with the parent merger.

If the parent merger is completed and if BOCH’s former shareholders sell substantial amounts of Columbia common shares in the public market following completion of the parent merger, the market price of Columbia common shares may decrease. These sales might also make it more difficult for Columbia to sell equity or equity-related securities at a time and price that it otherwise would deem appropriate.

The combined company may fail to realize the anticipated benefits of the mergers.

The success of the mergers will depend on, among other things, the combined company’s ability to combine the businesses of Columbia and BOCH. If the combined company is not able to successfully achieve this objective, the anticipated benefits of the mergers may not be realized fully, or at all, or may take longer to realize than expected.

Columbia and BOCH have operated and, until the consummation of the parent merger, will continue to operate independently. It is possible that the integration process or other factors could result in the loss or departure of key employees, the disruption of the ongoing business of Columbia or BOCH or inconsistencies in standards, controls, procedures and policies. It is also possible that clients, customers, depositors and

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counterparties of Columbia or BOCH could choose to discontinue their relationships with the combined company because they prefer doing business with an independent company or for any other reason, which would adversely affect the future performance of the combined company. These transition matters could have an adverse effect on each of Columbia and BOCH during the pre-merger period and for an undetermined amount of time after the consummation of the mergers.

Certain BOCH directors and executive officers may have interests in the parent merger different from the interests of BOCH shareholders.

In considering the recommendations of the BOCH board of directors, BOCH shareholders should be aware that certain directors and executive officers of BOCH have interests in the mergers that may differ from, or may be in addition to, the interests of BOCH shareholders generally. The BOCH board of directors was aware of these interests and considered them, among other matters, when it adopted the merger agreement and in making its recommendations that the BOCH shareholders approve the principal terms of the merger agreement. These interests include:

For a more complete description of the interests of BOCH’s directors and executive officers in the mergers, see “The Mergers—Interests of BOCH’s Directors and Executive Officers in the Mergers.”

Risk Factors Relating to BOCH and BOCH’s Business

BOCH is, and will continue to be, subject to the risks described in BOCH’s Annual Report on Form 10-K for the fiscal year ended December 31, 2020, as updated by subsequent Quarterly Reports on Form 10-Q and Current Reports on Form 8-K, all of which are filed with the SEC and incorporated by reference into this proxy statement/prospectus. See “Documents Incorporated by Reference” and “Where You Can Find More Information” included elsewhere in this proxy statement/prospectus.

Risk Factors Relating to Columbia and Columbia’s Business

Columbia is, and will continue to be, subject to the risks described in Columbia’s Annual Report on Form 10-K for the fiscal year ended December 31, 2020, as updated by subsequent Quarterly Reports on Form 10-Q and Current Reports on Form 8-K, all of which are filed with the SEC and incorporated by reference into this proxy statement/prospectus. See “Documents Incorporated by Reference” and “Where You Can Find More Information” included elsewhere in this proxy statement/prospectus.

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MARKET VALUE OF SECURITIES

Stock Prices

The following table sets forth the closing sale prices per Columbia common share and BOCH common share on June 22, 2021, the last trading day completed before the public announcement of the signing of the merger agreement, and on August 12, 2021, the latest practicable date before the date of this proxy statement/prospectus. This table also shows the implied value of the merger consideration to be issued in exchange for each share of BOCH common share, which was calculated by multiplying the closing price of Columbia common stock on those dates by the exchange ratio of 0.40.

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CAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTS

This document, including information included or incorporated by reference in this document, may contain forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. These forward-looking statements include, but are not limited to, (i) statements about the benefits of the mergers, including future financial and operating results, cost savings, enhancements to revenue and accretion to reported earnings that may be realized from the mergers; (ii) statements about our respective plans, objectives, expectations and intentions and other statements that are not historical facts; and (iii) other statements identified by words such as “expects,” “anticipates,” “intends,” “plans,” “believes,” “seeks,” “estimates,” “continues,” “prospects,” “projections,” or “potential,” future conditional verbs such as “will,” “would,” “should,” “could,” or “may” or words of similar meaning. These forward-looking statements are based on current beliefs and expectations of Columbia’s and BOCH’s managements and are inherently subject to significant business, economic and competitive uncertainties and contingencies, many of which are beyond Columbia’s and BOCH’s control. In addition, these forward-looking statements are subject to assumptions with respect to future business strategies and decisions that are subject to change.

In addition to factors previously disclosed in Columbia’s and BOCH’s reports filed with the SEC and those identified elsewhere in this filing (including the section entitled “Risk Factors”), the following potential factors, among others, could cause actual results to differ materially from the anticipated results or other expectations in the forward-looking statements:

All subsequent written and oral forward-looking statements concerning the proposed transactions or other matters attributable to Columbia or BOCH or any person acting on behalf of Columbia or BOCH are expressly qualified in their entirety by the cautionary statements above. Neither Columbia nor BOCH undertakes any obligation to update any forward-looking statements to reflect circumstances or events that occur after the date the forward-looking statements are made.

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THE MERGERS

The following is a discussion of the mergers and the material terms of the merger agreement between Columbia and BOCH. You are urged to read carefully the merger agreement in its entirety, a copy of which is attached as Appendix A to this document and incorporated by reference herein. This summary may not contain all of the information about the merger agreement that is important to you. We encourage you to read the merger agreement carefully and in its entirety. Factual information about Columbia and BOCH can be found elsewhere in this proxy statement/prospectus and in the public filings Columbia and BOCH make with the SEC, as described in the section entitled “Where You Can Find More Information.”

Terms of the Mergers

Transaction Structure

The merger agreement provides for the acquisition of BOCH by Columbia through the merger of BOCH with and into Columbia with Columbia continuing as the surviving corporation in the parent merger. Promptly following the parent merger, Merchants Bank will merge with and into Columbia Bank, with Columbia Bank continuing as the surviving bank in the bank merger.

Merger Consideration

In the merger, BOCH shareholders will have the right, with respect to each of their BOCH common shares, to receive a number of Columbia common shares equal to the exchange ratio, which is 0.40 of a share of Columbia common stock per share of BOCH common stock, subject to any adjustments set forth in the merger agreement.

The exchange ratio adjustment mechanisms provide that, in the event that (a) the Columbia average closing price is less than the signing price and (b) the Columbia average closing price underperforms the KBW Regional Banking Index by greater than 15%, then BOCH can terminate the merger agreement unless Columbia, at its option and in its sole discretion, adjusts the exchange ratio to a number equal to the lesser of (A) a quotient, the numerator of which is the signing price multiplied by the exchange ratio, and the denominator of which is the Columbia average closing price, or (B) the product of (1) a quotient, the numerator of which is the signing price multiplied by the exchange ratio, and the denominator of which is the Columbia average closing price, multiplied by (2) the percent change in the KBW Regional Banking Index.

The value of the Columbia common shares to be received by BOCH shareholders in the parent merger may vary from the value as of the date we announced the mergers, the date that this document was mailed to BOCH shareholders, the date of the meeting of BOCH shareholders and the determination period. Any change in the market price of Columbia common shares prior to completion of the parent merger will affect the value of the merger consideration that BOCH shareholders will receive upon completion of the parent merger. Accordingly, at the time of the BOCH special meeting, BOCH shareholders will not know or be able to calculate the value of the per share consideration they would receive upon completion of the parent merger.

Adjustment for Transaction Expenses

If, immediately prior to the effective time of the parent merger, the transaction expenses incurred by BOCH exceed the threshold amount, there will be an adjustment made to the exchange ratio such that the excess amount of transaction expenses will be deducted from the aggregate merger consideration. In that event, the exchange ratio will be equal to the product of (x) the exchange ratio as calculated in accordance with the merger agreement and (y) the quotient obtained by dividing (1) the aggregate merger consideration minus the reduction amount by (2) the aggregate merger consideration. In the event of a transaction or series of related transactions in which a person acquires record or beneficial ownership of Columbia common shares that, together with stock held by such person, constitutes greater than 50% of the outstanding Columbia common shares, the reduction amount will be zero.

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Treatment of BOCH Equity Awards

BOCH Options

At the effective time of the parent merger, each outstanding BOCH option, whether vested or unvested, will, automatically and without any action on the part of the holder thereof, be cancelled and will only entitle the holder of such BOCH option to receive (without interest), no later than the first payroll period following the effective time of the merger and in any event no later than 30 calendar days following the closing date, an amount in cash equal to the product of (x) the number of shares subject to such BOCH options multiplied by (y) the excess, if any, of (A) the equity award cashout price over (B) the exercise price per share of such BOCH option, less applicable taxes required to be withheld with respect to such payment. Any BOCH option which has an exercise price per share that is greater than or equal to the equity award cashout price will be cancelled at the effective time of the parent merger for no consideration or payment.

Restricted Stock Awards

At the effective time of the parent merger, each outstanding BOCH restricted stock award will, automatically and without any action on the part of the holder thereof, accelerate in full and will be converted into, and become exchanged for the merger consideration (less applicable taxes required to be withheld with respect to such vesting).

Conversion of Shares; Exchange of Certificates

The conversion of BOCH common shares (other than treasury shares) into the right to receive the merger consideration will occur automatically at the effective time. As soon as reasonably practicable after the effective time, the exchange agent will exchange certificates representing BOCH common shares (other than treasury shares) for merger consideration to be received in the merger pursuant to the terms of the merger agreement.

Letters of Transmittal

As soon as reasonably practicable after the completion of the parent merger, but in any event within five business days thereafter, the exchange agent will send a letter of transmittal and instructions for surrendering certificates or book-entry shares in exchange for the merger consideration, any cash in lieu of fractional shares of Columbia (as described below), and any dividends or distributions to which a holder may be entitled (as described below), to each holder of record of certificates or book-entry shares which, immediately prior to the completion of the parent merger, represented BOCH common shares, whose shares were converted into the right to receive the merger consideration.

If a certificate for BOCH common shares has been lost, stolen or destroyed, the exchange agent will issue the consideration properly payable under the merger agreement upon receipt of an affidavit as to that loss, theft or destruction and, if requested by the exchange agent, the posting of a bond to indemnify the exchange agent against any claim that may be made against it with respect to such certificate.

Cash in Lieu of Fractional Shares

No fractional Columbia common shares will be issued upon the surrender of certificates or book-entry shares for exchange, and no dividend or distribution with respect to Columbia common shares will be payable on or with respect to any fractional share, and such fractional share interests will not entitle the owner thereof to vote or to any other rights of a shareholder of Columbia. In lieu of the issuance of any such fractional share, Columbia will pay to each former shareholder of BOCH who otherwise would be entitled to receive such fractional share an amount in cash (rounded to the nearest cent) determined by multiplying (i) the Columbia average closing price for the determination period by (ii) the fraction of a share (after taking into account all shares of BOCH common stock held by such holder at the effective time of the parent merger and rounded to the nearest thousandth when expressed in decimal form) of Columbia common shares which such holder would otherwise be entitled to receive.

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Dividends and Distributions

Until certificates or book-entry shares representing BOCH common shares are surrendered for exchange, any dividends or other distributions with a record date after the effective time of the parent merger with respect to Columbia common shares into which such BOCH common shares may have been converted will not be paid. Following surrender of any such certificates or book-entry shares, the record holder thereof will be entitled to receive, without interest, any dividends or other distributions with a record date after the effective time of the parent merger payable with respect to the whole number of Columbia common shares represented by such certificates or book-entry shares and paid prior to the surrender date, and at the appropriate payment date, the amount of dividends or other distributions payable with respect to Columbia common shares represented by such certificates or book-entry shares with a record date after the effective time of the parent merger but before the surrender date and with a payment date after the issuance of Columbia common shares issuable with respect to such certificates or book-entry shares.

After the effective time of the parent merger, there will be no transfers on the share transfer books of BOCH of any BOCH common shares. If certificates representing such shares are presented for transfer after the completion of the parent merger, they will be cancelled and exchanged for the merger consideration into which the shares represented by that certificate have been converted.

Dissenting Shares

Under California law, BOCH shareholders will not be entitled to exercise any appraisal or dissenters’ rights in connection with any of the proposals being presented to them.

Regulatory Approvals Required for the Mergers

Each of Columbia and BOCH has agreed to use its commercially reasonable efforts to obtain all regulatory approvals required to complete the mergers and the other transactions contemplated by the merger agreement. These approvals include approval from the Federal Reserve, the FDIC and the WDFI, among others. As of the date of this proxy statement/prospectus, Columbia and BOCH have submitted applications and notifications to obtain the required regulatory approvals.

Federal Deposit Insurance Corporation

The prior approval of the FDIC will be required under Section 18(c) of the Federal Deposit Insurance Act, which we refer to as the Bank Merger Act, to merge Merchants Bank with and into Columbia Bank. In evaluating an application filed under the Bank Merger Act, the FDIC generally considers: (1) the competitive impact of the transaction, (2) financial and managerial resources of the banks party to the bank merger, (3) the banks’ effectiveness in combating money-laundering activities and (4) the extent to which the bank merger or mergers would result in greater or more concentrated risks to the stability of the United States banking or financial system. The FDIC also reviews the performance records of the relevant depository institutions under the Community Reinvestment Act of 1997, which we refer to as CRA, including their CRA ratings. In connection with its review under the Bank Merger Act, the FDIC will provide an opportunity for public comment on the application for the bank merger and is authorized to hold a public meeting or other proceeding if it determines that would be appropriate.

Transactions approved by the FDIC generally may not be completed until 30 days after the approval of the FDIC is received, during which time the Department of Justice, which we refer to as the DOJ, may challenge the transaction on antitrust grounds. With the approval of the FDIC and the concurrence of the DOJ, the waiting period may be reduced to no less than 15 days. The commencement of an antitrust action would stay the effectiveness of such an approval unless a court specifically ordered otherwise. In reviewing the bank merger, the DOJ could analyze the bank merger’s effect on competition differently than the FDIC, and thus it is possible that the DOJ could reach a different conclusion than the FDIC does regarding the bank merger’s effects on competition. A determination by the DOJ not to object to the bank merger may not prevent the filing of antitrust actions by private persons or state attorneys general.

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Federal Reserve Board

The transactions contemplated by the merger agreement are subject to approval by the Federal Reserve pursuant to the Bank Holding Company Act of 1956, as amended, which we refer to as the BHC Act. Columbia has submitted an application pursuant to the BHC Act and Regulation Y seeking the prior approval of the Federal Reserve for Columbia to acquire BOCH. In considering the approval of transactions such as the mergers, the Federal Reserve is required by the BHC Act to review, with respect to the bank holding companies and the banks concerned: (1) the competitive impact of the transaction, (2) the financial condition and future prospects, including capital positions and managerial resources, (3) the convenience and needs of the communities to be served and the record of the insured depository institution subsidiaries of the bank holding companies under the CRA, (4) the effectiveness of the companies and the depository institutions concerned in combating money laundering activities and (5) the extent to which the proposal would result in greater or more concentrated risks to the stability of the United States banking or financial system. In connection with such a review, the Federal Reserve will provide an opportunity for public comment on the application and is authorized to hold a public meeting or other proceeding if it determines such meeting or other proceeding would be appropriate. The Federal Reserve may not approve a proposal that would have significant adverse effects on competition or on the concentration of resources in any banking market.

Additional Regulatory Approvals and Notices

The transactions contemplated by the merger agreement are also subject to approval by or other filing to the California Department of Financial Protection and Innovation and the WDFI, and notifications may be filed with various other regulatory agencies.

There can be no assurances that such approvals will be received on a timely basis, or as to the ability of Columbia and BOCH to obtain the approvals on satisfactory terms or the absence of litigation challenging such approvals. There can likewise be no assurances that federal or state regulatory authorities will not attempt to challenge the mergers on antitrust grounds or for other reasons, or, if such a challenge is made, as to the result of such challenge. The parties’ obligations to complete the transactions contemplated by the merger agreement are subject to a number of conditions, including the receipt of all requisite regulatory approvals.

Accounting Treatment

In accordance with current accounting guidance, the mergers will be accounted for using the acquisition method. The result of this method is that the assets and liabilities of Columbia will be carried forward at their recorded amounts, Columbia’s historical operating results will be unchanged for the prior periods being reported on and the assets and liabilities of BOCH will be adjusted to fair value at the date of the mergers. In addition, all identified intangibles will be recorded at fair value and included as part of the net assets acquired. To the extent that the purchase price, consisting of cash plus the number of Columbia common shares to be issued to former BOCH shareholders at fair value, exceeds the fair value of the net assets acquired, including identifiable intangible assets, from BOCH at the date of the mergers, that amount will be reported as goodwill. In accordance with current accounting guidance, goodwill will not be amortized but, in general, will be evaluated for impairment annually. Identified intangibles will be amortized over their estimated lives. Further, the acquisition method of accounting results in the operating results of BOCH being included in the operating results of Columbia beginning from the date of completion of the mergers.

Public Trading Markets

Columbia common shares are listed on the Nasdaq Global Select Market under the symbol “COLB.” BOCH common shares are listed on the Nasdaq Global Market under the symbol “BOCH.” Upon completion of the parent merger, BOCH common shares will be delisted from the Nasdaq Global Market and thereafter will be deregistered under the Securities Exchange Act of 1934, as amended, which we refer to as the Exchange Act. The Columbia common shares issued in the parent merger will be listed on the Nasdaq Global Select Market.

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Resale of Columbia Common Shares

All Columbia common shares received by BOCH shareholders in the parent merger will be freely tradable for purposes of the Securities Act and the Exchange Act, except for Columbia common shares received by any such holder who becomes an “affiliate” of Columbia after completion of the parent merger. This document does not cover resales of Columbia common shares received by any person upon completion of the parent merger, and no person is authorized to make any use of this document in connection with any resale.

Background of the Mergers

Over the last several years, BOCH’s board of directors and executive management has regularly reviewed and discussed BOCH’s business, performance, prospects and long-term strategy in the context of enhancing long-term value for all BOCH’s shareholders. In recent years, these strategic discussions have been set against a backdrop of, among other things, the impact of low interest rates and the challenging regulatory, compliance and competitive environments facing financial institutions generally, as well as conditions and ongoing consolidation in the financial services industry, including the challenges posed to banks of BOCH’s size operating principally in Northern California. The BOCH board of directors has considered, from time to time, various potential strategic alternatives, including transactions involving other financial institutions, such as potential acquisitions of financial institutions of a smaller size, mergers with financial institutions of similar size or business combinations with larger financial institutions. Furthermore, BOCH had on occasion been approached by various financial institutions to discuss the potential of combining organizations and BOCH had approached other entities about combining with them. Columbia’s management and board of directors also regularly reviews its business strategies and opportunities, including opportunities to expand its geographic footprint, with a longer-term view of further enhancing the value of the Columbia franchise through acquisitions.

In the fourth quarter of 2020, the BOCH board of directors discussed the challenges and competitive landscape in the banking industry, as well as the overall market, economic and regulatory environments. The BOCH board of directors also discussed in general the standalone financial forecast of BOCH and the merits of continuing operations on a standalone basis and determined to explore a potential process to combine with a larger financial institution. The BOCH board or directors then embarked on an extensive educational process on various strategic alternatives with the advice and input of its financial advisor and counsel.

On January 19, 2021, the BOCH board of directors held a meeting that was also attended by representatives of Raymond James, a nationally-recognized investment banking firm with substantial experience in transactions similar to the proposed mergers. At this meeting, Raymond James representatives reviewed and discussed (i) the current market environment for community banks in general, (ii) the current merger and acquisition environment within the banking industry, (iii) the current day-to-day market valuation for BOCH, (iv) the standalone financial forecast of BOCH and the merits of continuing operations on a standalone basis, and (v) various strategic alternatives for BOCH, including the prospects of BOCH finding a potential partner.

On February 16, 2021, representatives of Miller Nash reviewed with the BOCH board of directors the fiduciary duties of directors in connection with their consideration of a proposed business combination and the likely terms of a merger agreement. At this meeting, the BOCH board of directors approved the engagement of Raymond James to serve as financial advisor to BOCH in a potential sale or merger transaction. Raymond James was subsequently engaged to act as financial advisor to BOCH in a potential sale or merger transaction.

On March 16, 2021, the BOCH board of directors convened a meeting that representatives of Raymond James and Miller Nash also attended. At the meeting, Raymond James representatives provided an update on market conditions and reviewed publicly available information regarding an inventory of potential merger partners, including their trading multiples, for consideration by the BOCH board of directors. After considering the market conditions and other factors, the BOCH board of directors decided to launch a process to explore a combination with a larger financial institution in early April, once first quarter 2021 results became available to be provided confidentially to interested parties that agreed to execute non-disclosure agreements. After reviewing

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an inventory of potential merger partners and considering the potential risks associated with a potential business combination, including the disclosure of confidential information to competitors and the consequences of an abandoned transaction to BOCH shareholders, employees and customers, and following discussion with Raymond James regarding the inventory and those financial institutions likely to have interest in a proposed combination, the BOCH board of directors selected six potential merger partners to be contacted in regards to a potential transaction. The BOCH board of directors decided these potential merger partners possessed the highest capacity to provide a premium to BOCH shareholders in a transaction as well as the strongest cultural and operational match, based on publically available information. Prior to first quarter 2021 results becoming available, the BOCH board of directors directed Raymond James to contact a few selected banks that could be interested to arrange introductory meetings with Randall S. Eslick, President and Chief Executive Officer of BOCH.

Mr. Eslick had in-person meetings facilitated by Raymond James with Clint Stein from Columbia on April 1, 2021. These meetings were informal and not specific to the merger but rather focused on providing an update regarding publicly available information as to the financial condition, general business interests, and operating strategy of BOCH and of each party.

On April 13, 2021, at the direction of BOCH, Raymond James began to contact potential merger partners, including Columbia, as part of the process described above. After signing a non-disclosure agreement, potential merger partners were provided a confidential information memorandum, granted access to a due diligence data room and offered diligence calls with BOCH management. During the month of April, Raymond James contacted six parties, four of which executed non-disclosure agreements and three of which met with BOCH management. Columbia and BOCH executed a non-disclosure agreement on April 16, 2021, and due diligence discussions ensued between Columbia and BOCH.

On April 27, 2021, Clint Stein, President and Chief Executive Officer of Columbia, met with the mergers and acquisitions committee of the Columbia board of directors, together with representatives of Columbia’s financial advisor, Keefe, Bruyette & Woods, Inc., which we refer to as KBW. After discussion concerning BOCH and a potential combination transaction, the committee authorized management to submit a non-binding indication of interest. On May 7, 2021, BOCH received two non-binding indications of interest and one verbal indication that a third non-binding indication would be delivered the following week.

On May 14, 2021, the BOCH board of directors convened a meeting to consider the three non-binding indications of interest. Representatives from Raymond James and Miller Nash attended this meeting. Columbia submitted a non-binding indication of interest to exchange 100% stock consideration in a fixed exchange of 0.40 of a share of Columbia common stock for each BOCH share, representing a market price of $17.85 per BOCH share. The closing price of BOCH stock on May 13, 2021 was $14.00. The second indication of interest was from Party A for $16.17 per BOCH share in all stock or with a mix of 90% stock and 10% cash and a willingness to consider alternate consideration mixtures. The final indication of interest was from Party B for an all-stock transaction with an unspecified exchange ratio at a stated value of $15.15 per BOCH share. In order to move to a due diligence phase, each party requested a period of exclusivity. Columbia indicated that BOCH would need to grant it exclusivity for 60 days. After reviewing the three non-binding indications of interest, in consultation with Raymond James, the BOCH board of directors instructed Raymond James to inform Columbia that it was chosen subject to the receipt of a revised non-binding indication (i) further clarifying any material conditions to closing and, (ii) reducing the exclusivity period from 60 to 45 days.

On May 14, 2021, at the instruction of the BOCH board of directors, Raymond James and CEO Eslick informed Parties A and B that BOCH had decided to go a different direction. CEO Eslick and a representative of Raymond James informed Columbia and a representative of KBW, respectively, that BOCH had decided to move forward with Columbia subject to receipt of a revised non-binding indication as described above. After consulting with Columbia, a representative of KBW informed a representative of Raymond James that Columbia would provide a revised non-binding indication of interest as requested and on May 14, 2021, Columbia and BOCH signed the non-binding indication of interest and agreed to move forward to the due diligence phase.

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Promptly following the execution of the non-binding indication of interest, including a 45-day exclusivity agreement, Columbia commenced its due diligence on BOCH and BOCH commenced its reverse due diligence on Columbia, including a meeting between BOCH executive management and Columbia’s management team, held in Tacoma, Washington on May 25, 2021.

On May 19, 2021, Columbia management, together with representatives of KBW, updated the Columbia mergers and acquisitions committee of the Columbia Board and the Columbia board regarding the status of discussions with BOCH and discussed, among other things, the strategic rationale for the proposed transaction and estimated timeline.

On June 4, 2021, representatives of Sullivan & Cromwell LLP, counsel to Columbia, sent an initial draft of the merger agreement to representatives of Miller Nash. On June 6, 2021, representatives of Sullivan & Cromwell sent to representatives of Miller Nash a draft employment agreement for Mr. Eslick to be effective upon closing of the parent merger. On June 10, 2021, representatives of Miller Nash sent to representatives of Sullivan & Cromwell an initial mark-up of the draft merger agreement, and on June 11, 2021, representatives of Sullivan & Cromwell sent to representatives of Miller Nash a draft of a proposed voting agreement for the potential transaction that Columbia required be entered into with members of the BOCH board of directors. Thereafter, until the execution of the merger agreement on June 23, 2021, the parties and their respective legal advisors exchanged drafts of, and engaged in discussion and negotiations concerning the terms of, the merger agreement, voting agreements and employment agreement. Key items negotiated included, among others, certain of the representations and warranties to be provided by BOCH and related thresholds and materiality qualifications, certain restrictive covenants to be applicable to BOCH between signing and closing, and the amount of transaction-related expenses BOCH could incur without an impact on the exchange ratio. During this time, Columbia and BOCH continued their extensive documentary and other due diligence in parallel with the negotiation of the transaction documentation.

On June 23, 2021, the BOCH board of directors held a meeting at which representatives of the BOCH executive management team, Raymond James and Miller Nash were present. Management presented its final due diligence report to the BOCH board of directors. Also at the meeting, Raymond James reviewed the financial aspects of the proposed merger and rendered to the BOCH board of directors an opinion to the effect that, as of such date and subject to the procedures followed, assumptions made, matters considered, and qualifications and limitations on the review undertaken by Raymond James as set forth in its opinion, the consideration contemplated in the proposed merger was fair, from a financial point of view, to the holders of BOCH common stock. Representatives of Miller Nash reviewed with the BOCH board of directors their fiduciary duties in connection with their consideration of the proposed business combination and the terms of the merger agreement. After extensive discussion regarding the terms of the merger agreement and the voting agreements, a full analysis of BOCH’s reasons for engaging in the proposed business combination with Columbia, including those set forth below under “The Mergers—Recommendations of the BOCH Board of Directors and Reasons for the Merger,” and consideration of other relevant issues, including a variety of business, financial and market factors, the BOCH board of directors unanimously adopted and approved the merger agreement and the mergers.

On June 23, 2021, the Columbia board of directors also held a meeting to discuss the proposed transaction. Also in attendance at the meeting were representatives of management, KBW and Sullivan & Cromwell. Among other things, Mr. Stein and other members of management discussed the terms of the proposed transaction, the results of Columbia’s due diligence investigation of BOCH and the strategic rationale for the transaction, and representatives of KBW presented on certain financial aspects of the transaction. A representative of Sullivan & Cromwell discussed with the members of the Columbia board of directors their fiduciary duties in connection with their consideration of the proposed transaction and reviewed for the Board the terms of the draft definitive merger agreement and related agreements. After extensive discussion regarding the terms of the merger agreement and the related agreements, and taking into consideration the matters described under “The Merger—Columbia Board of Directors’ Reasons for the Mergers” among others, the Columbia board of directors unanimously approved the merger agreement and the mergers.

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Following the BOCH board of directors meeting on June 23, 2021 and Columbia’s board of directors meeting on June 23, 2021, BOCH and Columbia entered into the merger agreement and announced the mergers on June 23, 2021. The parties to the voting agreements also entered into the voting agreements, and Mr. Eslick and Columbia entered into his employment agreement to be effective upon closing of the parent merger.

Columbia Board of Directors’ Reasons for the Mergers

In reaching its decision to adopt and approve the merger agreement, the Columbia board of directors evaluated the mergers in consultation with Columbia’s management, as well as Columbia’s legal and financial advisors, and considered a number of factors, including:

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The foregoing discussion of the factors considered by the Columbia board of directors is not intended to be exhaustive, but, rather, includes the material factors considered by the Columbia board of directors. In reaching its decision to adopt and approve the merger agreement, and the other transactions contemplated by the merger agreement, the Columbia board of directors did not quantify or assign any relative weights to the factors considered, and individual directors may have given different weights to different factors. The Columbia board of directors considered all these factors as a whole and overall considered the factors to be favorable to, and to support, its determination.

Recommendations of the BOCH Board of Directors and Reasons for the Mergers

In reaching its decision to approve the merger agreement and the transactions contemplated thereby, the BOCH board of directors evaluated the merger agreement in consultation with executive management, as well as its legal counsel and financial advisor, and considered numerous factors, including the following:

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The foregoing discussion of the factors considered by the BOCH board of directors is not intended to be exhaustive, but, rather, includes the material factors considered by the BOCH board of directors. In reaching its decision to adopt and approve the merger agreement, and the other transactions contemplated by the merger agreement, the BOCH board of directors did not quantify or assign any relative weights to the factors considered, and individual directors may have given different weights to different factors. The BOCH board of directors considered all these factors as a whole, including discussions with, and questioning of, BOCH’s management and its financial and legal advisors, and overall considered the factors to be favorable to, and to support, its determination. The BOCH board of directors also relied on the experience of Raymond James, its financial advisor, for analyses of the financial terms of the mergers and for its opinion as to the fairness, from a financial point of view, of the exchange ratio to the shareholders of BOCH.

For the reasons set forth above, the BOCH board of directors unanimously determined that the mergers, the merger agreement and the transactions contemplated by the merger agreement are advisable and in the best interests of BOCH and its shareholders, and approved and adopted the merger agreement. The BOCH board of directors unanimously recommends that the BOCH shareholders vote “FOR” the merger proposal, “FOR” the merger-related named executive officer compensation proposal, and “FOR” the adjournment proposal.

Opinion of Raymond James

BOCH retained Raymond James as its financial advisor on April 8, 2021. As part of that engagement, the BOCH board of directors requested that Raymond James evaluate the fairness, from a financial point of view, to the holders of BOCH common shares of the merger consideration to be received by such holders pursuant to the merger agreement.

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At the June 23, 2021 meeting of the BOCH board of directors, representatives of Raymond James rendered its oral opinion, which was subsequently confirmed by delivery of the Raymond James opinion, as to the fairness, as of such date, from a financial point of view, to the holders of BOCH common shares of the merger consideration to be received by such holders pursuant to the merger agreement, based upon and subject to the qualifications, assumptions and other matters considered in connection with the preparation of its opinion.

The full text of the Raymond James opinion is attached as Appendix C to this document. The summary of the Raymond James opinion set forth in this document is qualified in its entirety by reference to the full text of such written opinion. Holders of BOCH common shares are urged to read this opinion in its entirety.

Raymond James provided the Raymond James opinion for the information of the BOCH board of directors (solely in its capacity as such) in connection with, and for purposes of, its consideration of the parent merger and the Raymond James opinion only addresses whether the merger consideration to be received by the holders of the BOCH common shares pursuant to the merger agreement was fair, from a financial point of view, to such holders. The Raymond James opinion does not address any other term or aspect of the merger agreement or the mergers contemplated thereby. The Raymond James opinion does not constitute a recommendation to the BOCH board of directors or to any holder of BOCH common shares as to how the BOCH board of directors, such shareholder, or any other person should vote or otherwise act with respect to the mergers or any other matter. Raymond James does not express any opinion as to the likely trading range of Columbia common shares following the mergers, which may vary depending on numerous factors that generally impact the price of securities or on the financial condition of Columbia at that time.

In connection with its review of the proposed mergers and the preparation of the Raymond James opinion, Raymond James, among other things:

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With BOCH’s consent, Raymond James assumed and relied upon the accuracy and completeness of all information supplied by or on behalf of BOCH, or otherwise reviewed by or discussed with Raymond James. Raymond James did not undertake any duty or responsibility to independently verify any of such information. Furthermore, Raymond James undertook no independent analysis of any potential or actual litigation, regulatory action, possible unasserted claims or other contingent liabilities, to which BOCH or Columbia is a party or may be subject, or of any governmental investigation of any possible unasserted claims or other contingent liabilities to which BOCH or Columbia is a party or may be subject. With BOCH’s consent, the Raymond James opinion made no assumption concerning, and therefore did not consider, the potential effects of any such litigation, claims or investigations or possible assertions. Raymond James did not make or obtain an independent appraisal of BOCH’s assets or liabilities (contingent or otherwise). With respect to the projections and any other information and data provided to or otherwise reviewed by or discussed with Raymond James, Raymond James, with BOCH’s consent, assumed that the projections and such other information and data were reasonably prepared in good faith on bases reflecting the best currently available estimates and judgments of BOCH’s management and Raymond James relied on BOCH to advise Raymond James promptly if any information previously provided became inaccurate or was required to be updated during the period of its review. Raymond James expressed no opinion with respect to the projections or the assumptions on which they were based. Based upon the terms specified in the merger agreement, Raymond James assumed that the merger will qualify as a reorganization under the provisions of Section 368(a) of the Code. Raymond James relied upon and assumed, without independent verification, that the final form of the merger agreement would be substantially similar to the draft agreement reviewed by Raymond James in all respects material to its analysis, and that the mergers would be consummated in accordance with the terms of the merger agreement without waiver of or amendment of any of the conditions thereto. Furthermore, Raymond James assumed, in all respects material to its analysis, that the representations and warranties of each party contained in the merger agreement were true and correct and that each party will perform all of the covenants and agreements required to be performed by it under the merger agreement without being waived. Raymond James also relied upon and assumed, without independent verification, that (i) the mergers will be consummated in a manner that complies in all respects with all applicable federal and state statutes, rules and regulations, and (ii) all governmental, regulatory or other consents and approvals necessary for the consummation of the mergers will be obtained and that no delay, limitations, restrictions, or conditions would be imposed or amendments, modifications, or waivers made that would have an effect on the mergers or that would be material to Raymond James’ analysis or opinion.

Raymond James expressed no opinion as to the underlying business decision to effect the mergers, the structure or tax consequences of the mergers, or the availability or advisability of any alternatives to the mergers. The Raymond James opinion is limited to the fairness, from a financial point of view, of the merger consideration to be received by the holders of BOCH common shares. Raymond James expressed no opinion with respect to any other reasons (legal, business, or otherwise) that may support the decision of the BOCH board of directors to approve or consummate the mergers. Furthermore, no opinion, counsel or interpretation was intended by Raymond James on matters that require legal, accounting or tax advice. Raymond James assumed that such opinions, counsel, or interpretations had been or would be obtained from appropriate professional sources. Furthermore, Raymond James relied, with BOCH’s consent, on the fact that BOCH was assisted by legal, accounting, and tax advisors and, with BOCH’s consent relied upon and assumed the accuracy and completeness of the assessments by BOCH and its advisors as to all legal, accounting, and tax matters with respect to BOCH and the mergers.

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In formulating the Raymond James opinion, Raymond James considered only the merger consideration to be received by the holders of BOCH common shares, and Raymond James did not consider, and the Raymond James opinion did not address, the fairness of the amount or nature of any compensation to be paid or payable to any of BOCH’s officers, directors, or employees, or such class of persons, in connection with the mergers whether relative to the merger consideration or otherwise. Raymond James was not requested to opine as to, and the Raymond James opinion did not express an opinion as to or otherwise address, among other things: (1) the fairness of the mergers to the holders of any class of securities, creditors or other constituencies of BOCH, or to any other party, except and only to the extent expressly set forth in the last sentence of the Raymond James opinion or (2) the fairness of the mergers to any one class or group of BOCH or any other party’s security holders or other constituents vis-à-vis any other class or group of BOCH or such other party’s security holders or other constituents (including, without limitation, the allocation of any consideration to be received in the mergers amongst or within such classes or groups of security holders or other constituents). Raymond James expressed no opinion as to the impact of the mergers on the solvency or viability of BOCH or Columbia or the ability of BOCH or Columbia to pay their respective obligations when they become due.

Material Financial Analyses

The following summarizes the material financial analyses reviewed by Raymond James with the BOCH board of directors at its meeting on June 23, 2021, which material was considered by Raymond James in rendering the Raymond James opinion. No company or transaction used in the analyses described below is identical or directly comparable to BOCH or the contemplated transaction.

Selected Companies Analysis. Raymond James reviewed certain data for selected companies with publicly traded equity securities that it deemed relevant for its analysis. The selected groups represent companies Raymond James believed relevant to BOCH. Raymond James analyzed the relative valuation multiples of eight publicly-traded financial institutions that satisfied the following criteria: (i) traded over the NASDAQCM, NASDAQGM, NASDAQGS, NYSE, or NYSEAM stock exchanges; (ii) headquartered in Northern California; and (iii) had total assets between $1.0 billion and $5.0 billion. This group excluded targets of announced mergers. The selected companies that Raymond James deemed relevant included the following:

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Raymond James calculated various financial multiples for each selected public company, including price per share at close on June 22, 2021, compared to (i) tangible book value per share at the most recently reported financial period ended March 31, 2021 or December 31, 2020, as shown by S&P Global Market Intelligence; (ii) earnings per share for the latest 12 months available; (iii) consensus forward operating earnings per share, which we refer to as ‘21E EPS, for the 2021 fiscal year based on mean S&P Global Market Intelligence data; and (iv) consensus forward operating earnings per share, which we refer to as ‘22E EPS, for the 2022 fiscal year based on mean S&P Global Market Intelligence data. Raymond James reviewed the 75th percentile, mean, median and 25th percentile relative valuation multiples of the selected public companies and compared them to corresponding valuation multiples for BOCH implied by the merger consideration. The results of the selected public companies analysis are summarized below:

Furthermore, Raymond James applied the 75^th percentile, median, mean and 25^th percentile relative valuation multiples for each of the metrics to BOCH’s actual financial results to derive an implied transaction value. Raymond James then compared those implied values to $16.70, the value attributed to the per share merger consideration for the purposes of the Raymond James opinion. The results of this analysis are summarized below:

Selected Transaction Analysis. Raymond James analyzed publicly available information relating to selected regional transactions announced since March 1, 2020, involving financial institution targets headquartered in the Western United States, which includes the states of Alaska, Arizona, California, Hawaii, Idaho, Montana, Nevada, Oregon, Washington, and Wyoming, with assets between $1.0 billion and $5.0 billion. Raymond James also analyzed publicly available information relating to selected national transactions announced since March 1, 2020, involving financial institution targets headquartered in the United States with assets between $1.0 billion and $5.0 billion. Both regional and national selected transactions excluded: (i) transactions without publicly disclosed deal value or sufficient financial information; (ii) mergers of equals; (iii) transactions in which less than 100% of equity ownership was acquired; and (iv) transactions involving specialty finance companies. The selected transactions (with transaction announcement dates shown) used in the analysis included:

Regional Transactions (also listed under National Transactions):

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National Transactions:

Raymond James examined valuation multiples of transaction value compared to the target companies’ (i) most recent quarter tangible book value at announcement; (ii) last twelve months earnings at the time of announcement; and (iii) premium to core deposits (total deposits less time deposits greater than $100,000). Raymond James adjusted earnings of subchapter S corporations using an effective tax rate of 21%. Raymond James reviewed the 75th percentile, median, mean and 25th percentile relative valuation multiples of the selected regional and national transactions.

Regional Transactions:

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National Transactions:

Furthermore, Raymond James applied the 75^th percentile, median, mean and 25^th percentile relative valuation multiples for the selected regional and national transactions to BOCH’s tangible book value, last twelve months earnings, and core deposits. Raymond James then compared those implied values to $16.70, the value attributed to the per share merger consideration for the purposes of the Raymond James opinion. The results of the selected transactions analysis are summarized below:

Regional Transactions:

National Transactions:

Discounted Cash Flow Analysis. Raymond James analyzed the discounted present value of BOCH’s projected free cash flows for the nine months ending December 31, 2021 and the 12 months ending December 31, 2022 through December 31, 2025 on a standalone basis, which were provided to Raymond James and approved for its use by BOCH. Raymond James used tangible common equity in excess of a target ratio of 8.0% of tangible assets at the end of each projection period for free cash flow.

The discounted cash flow analysis was based solely on the projections. Consistent with the periods included in the projections, Raymond James used calendar year 2025 as the final year for the analysis and applied multiples, ranging from 13.0x to 15.0x, to calendar year 2025 adjusted net income in order to derive a range of terminal values for BOCH in 2025. The projected free cash flows and terminal values were discounted to present value using rates ranging from 11.0% to 13.0%. Raymond James arrived at its discount range by using the Modified CAPM (Capital Asset Pricing Model) methodology as presented in the 2020 Duff & Phelps Valuation Handbook.

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The resulting range of present equity values was divided by the number of diluted shares outstanding. Raymond James reviewed the range of per share prices derived in the discounted cash flow analysis and compared them to $16.70, the value attributed to the per share merger consideration for purposes of the Raymond James opinion. The results of the discounted cash flow analysis indicated a range of values from $13.62 per share to $16.06 per share.

In connection with its analysis, Raymond James considered and discussed with BOCH’s management how the discounted cash flow analysis would be affected by changes in the underlying assumptions. Raymond James noted that discounted cash flow analysis is a widely used valuation methodology, but the results of such methodology are highly dependent upon the numerous assumptions that must be made, and the results are not necessarily indicative of actual values or future results.

Additional Considerations. The preparation of a fairness opinion is a complex process and is not susceptible to a partial analysis or summary description. Raymond James believes that its analyses must be considered as a whole and that selecting portions of its analyses, without considering the analyses taken as a whole, would create an incomplete view of the process underlying the Raymond James opinion. In addition, Raymond James considered the results of all such analyses and did not assign relative weights to any of the analyses, but rather made qualitative judgments as to significance and relevance of each analysis and factor, so the ranges of valuations resulting from any particular analysis described above should not be taken to be the view of Raymond James as to BOCH’s actual value.

In performing its analyses, Raymond James made numerous assumptions with respect to industry performance, general business, economic and regulatory conditions and other matters, many of which are beyond BOCH’s control. The analyses performed by Raymond James are not necessarily indicative of actual values, trading values or actual future results which might be achieved, all of which may be significantly more or less favorable than suggested by such analyses. Such analyses were provided to the BOCH board of directors (solely in its capacity as such) and were prepared solely as part of the analysis by Raymond James of the fairness, from a financial point of view, to the holders of BOCH common shares of the merger consideration to be received by such holders in connection with the proposed merger pursuant to the merger agreement. The analyses do not purport to be appraisals or to reflect the prices at which companies may actually be sold, and such estimates are inherently subject to uncertainty. The Raymond James opinion was one of many factors taken into account by the BOCH board of directors in making its determination to approve the merger agreement. Neither the Raymond James opinion nor the analyses described above should be viewed as determinative of the views with respect to BOCH, Columbia, or the mergers of BOCH’s management or the BOCH board of directors. Raymond James provided advice to BOCH with respect to the proposed transaction. Raymond James did not, however, recommend any specific amount of consideration to the BOCH board of directors or indicate that any specific merger consideration constituted the only appropriate consideration for the mergers. BOCH placed no limits on the scope of the analysis performed, or opinion expressed, by Raymond James.

The Raymond James opinion was necessarily based upon market, economic, financial and other circumstances and conditions existing and disclosed to it on June 22, 2021, and any material change in such circumstances and conditions may affect the opinion of Raymond James, but Raymond James does not have any obligation to update, revise or reaffirm that opinion. Raymond James relied upon and assumed, without independent verification, that there had been no change in the business, assets, liabilities, financial condition, results of operations, cash flows or prospects of BOCH since the respective dates of the most recent financial statements and other information, financial or otherwise, provided to Raymond James that would be material to its analyses or the Raymond James opinion, and that there was no information or any facts that would make any of the information reviewed by Raymond James incomplete or misleading in any material respect.

As the BOCH board of directors was aware, there is significant uncertainty as to the potential direct and indirect business, financial, legal, economic and social implications and consequences of the coronavirus and associated illnesses and the actions and measures that countries, governments, regulatory agencies, central banks,

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international financing and funding organizations, stock markets, businesses and individuals have taken and may take to address the coronavirus and associated illnesses including, without limitation, those actions and measures pertaining to fiscal or monetary policies, legal and regulatory matters and the credit, financial and stock markets (collectively, the “Pandemic Effects”). Raymond James expressed no opinion or view as to the potential impact of the Pandemic Effects on its analysis, the Raymond James opinion, the mergers, Columbia, BOCH, or the value of the merger consideration after June 23, 2021.

As the BOCH board of directors was also aware, the credit, financial, and stock markets had been experiencing and do experience unusual volatility from time to time, and Raymond James expressed no opinion or view as to any potential effects of such volatility on the mergers, Columbia, or BOCH. The Raymond James opinion did not purport to address potential developments in any such credit, financial and stock markets on the value of the merger consideration.

Miscellaneous. During the two years preceding the date of the Raymond James opinion, Raymond James has been engaged by or otherwise performed the following services for BOCH, Columbia, or their respective subsidiaries, for which it was paid a fee (separately from any amounts that were paid to Raymond James under the engagement letter described in this proxy statement pursuant to which Raymond James was retained as a financial advisor to BOCH to assist in reviewing strategic alternatives): (i) Raymond James has provided services to BOCH in connection with a share repurchase program, for which Raymond James was paid commissions and may be paid commissions in the future; (ii) Raymond James has engaged in certain fixed income and other trading activities with Merchants Bank, for which Raymond James has earned income; and (iii) Raymond James has engaged in certain fixed income trading activity with Columbia Bank, for which Raymond James has earned income.

For services rendered in connection with the delivery of the Raymond James opinion, BOCH paid Raymond James a fee in the amount of $350,000 upon delivery of the Raymond James opinion. BOCH has also agreed to pay Raymond James a cash fee for advisory services in connection with the mergers equal to 1.35% of the aggregate merger consideration, payment of which is contingent upon the closing of the mergers. BOCH also agreed to reimburse Raymond James for its reasonable out-of-pocket expenses and disbursements incurred in connection with its services, including the fees and expenses of its counsel, and to indemnify Raymond James against certain liabilities arising out of its engagement.

Raymond James is actively involved in the investment banking business and regularly undertakes the valuation of investment securities in connection with public offerings, private placements, business combinations and similar transactions. In the ordinary course of business, Raymond James may trade in the securities of BOCH and Columbia for its own account and for the accounts of its customers and, accordingly, may at any time hold a long or short position in such securities. Raymond James may provide investment banking, financial advisory and other financial services to BOCH, Columbia, or other parties with interests in the mergers in the future, for which Raymond James may receive compensation.

Columbia Management and Board of Directors After the Mergers

Upon completion of the parent merger, the Columbia board of directors will consist of the directors serving on the Columbia board of directors prior to the effective time of the parent merger.

The current directors and senior executive officers of Columbia are expected to continue in their current positions, other than as has been or may be publicly announced by Columbia in the normal course of business. Information about the current Columbia directors and senior executive officers can be found in the documents listed under “Where You Can Find More Information” included elsewhere in this proxy statement/prospectus.

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Continuing Services as Directors on the Columbia Board

The Columbia board of directors after the mergers will include each of the current directors from the current Columbia board of directors. The Columbia board of directors presently consists of thirteen directors.

Continuing Employment with the Columbia Surviving Corporation

It is currently expected that the senior executive officers of Columbia will continue their employment with Columbia following the effective time of the parent merger on substantially similar terms and conditions in existence immediately prior to the effective time of the parent merger.

None of Columbia’s directors or senior executive officers is a party to, or participates in any, Columbia plan, program or arrangement that provides such director or senior executive officer with any kind of compensation that is based on or otherwise related to the completion of the mergers.

Directors’ and Officers’ Insurance

Columbia will continue to provide indemnification and insurance coverage to the directors and executive officers of Columbia.

Interests of BOCH’s Directors and Executive Officers in the Merger

In considering the recommendations of the BOCH board of directors, BOCH shareholders should be aware that certain directors and executive officers of BOCH have interests in the merger that may differ from, or may be in addition to, the interests of BOCH shareholders generally. These interests are described in more detail and quantified below. The BOCH board of directors was aware of these interests and considered them, among other matters, when it adopted the merger agreement and in making its recommendation that the BOCH shareholders approve the principal terms of the merger agreement. For purposes of all BOCH agreements and plans described below, the completion of the transactions contemplated by the merger agreement will constitute a change in control or a term of similar meaning.

Indemnification and Insurance

Under the merger agreement, Columbia will provide or purchase director and officer liability insurance for a period of six years following the effective time of the merger for purposes of reimbursing each present and former director and officer of BOCH or its subsidiaries with respect to claims arising from facts or events occurring before the effective time of the merger. Such insurance will provide for at least the same coverage and amounts, and contain terms and conditions no less advantageous to such persons, as the coverage currently provided by BOCH to the present and former directors and officers of BOCH or its subsidiaries immediately prior to the completion of the merger; provided that Columbia is not required to expend, in the aggregate for such six-year period, an amount in excess of 250% of the aggregate annual premiums paid by BOCH for any such insurance in place as of the date of the merger agreement. If any such annual expense at any time would exceed that amount, then Columbia will be required to maintain policies of insurance providing for the maximum coverage available at an annual premium equal to that amount. Notwithstanding and in lieu of the foregoing, BOCH will use its reasonable best efforts to purchase a six-year prepaid tail policy for directors’ and officers’ liability insurance on the terms described above and subject to certain other specifications agreed to by the parties, and fully pay for such policy prior to the effective time of the merger, at an aggregate cost up to but not exceeding 250% of the current annual premiums paid by BOCH for such insurance.

Treatment of BOCH Equity Awards

BOCH Options. BOCH has awarded options to purchase BOCH common stock to certain employees. At the effective time of the merger, each outstanding BOCH option, whether vested or unvested, will, automatically and

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without any action on the part of the holder thereof, be cancelled and will only entitle the holder of such BOCH option to receive (without interest), no later than the first payroll period following the effective time of the merger (and in any event no later than 30 calendar days following the closing date of the merger), an amount in cash equal to the product of (x) the number of shares covered by such BOCH options multiplied by (y) the excess, if any, of (A) the equity award cashout price over (B) the exercise price per share of such BOCH options, less applicable taxes required to be withheld with respect to such payment. As of the date of this proxy statement/prospectus, no executive officers and no directors held any unvested options.

BOCH Restricted Stock Awards. BOCH has granted awards of restricted stock to certain employees that are subject to vesting conditions. At the effective time of the merger, each outstanding share of restricted stock will, automatically and without any action on the part of the holder thereof, accelerate in full and will be converted into and exchanged for the merger consideration, less applicable taxes required to be withheld. For information concerning restricted stock held by certain BOCH executive officers, see “Merger-Related Compensation for BOCH’s Named Executive Officers” below. As of the date of this proxy statement/prospectus, no non-employee directors held any restricted stock awards.

Voting Support Agreements of BOCH Directors

In addition to any other restrictive covenant described below, in connection with the merger, each member of the BOCH board of directors was asked to, and did, enter into a voting support agreement pursuant to which he or she agreed, among other things, to vote his or her shares in favor of approval of (i) the principal terms of the merger agreement and the transactions contemplated thereby, (ii) any other matter that is required to facilitate the transactions contemplated by the merger agreement, and (iii) any proposal to adjourn or postpone the BOCH special meeting to a later date if there are not sufficient votes to approve the principal terms of the merger agreement. Such directors have also agreed to vote against any action or agreement submitted for approval to the shareholders of BOCH that would (i) result in a breach of any covenant, representation or warranty or any other obligation or agreement of BOCH under the merger agreement, (ii) result in any of the conditions to the consummation of the merger under the merger agreement not being fulfilled, (iii) be in competition with or opposition to the merger agreement or the merger, (iv) be an acquisition proposal or (v) impair the ability of Columbia to complete the merger, the ability of BOCH to complete the merger, or otherwise be inconsistent with, prevent, impede or delay the consummation of the transactions contemplated by the merger agreement.

The directors have also agreed, for a two-year period following the closing of the merger, not to directly or indirectly, for either him/her self or any other person, solicit, induce, or attempt to solicit or induce any employees or independent contractors, customers, business partners, or joint venturers of Columbia to participate in or transfer their business to a competing business, which is defined as any depository, wealth management, or trust business company or holding company thereof within the state of California, or to hire any person then employed by Columbia or who was employed by BOCH prior to the effective time of the merger, except that the foregoing prohibition does not apply to persons who were terminated more than 120 days beforehand and the former BOCH director did not solicit or otherwise interfere with or disrupt Columbia’s relationship with such person. See “The Merger Agreement—Related Agreements” included elsewhere in this proxy statement/prospectus. The form of voting support agreement is attached to this proxy statement/prospectus as Appendix B.

Employment Agreements of BOCH Executive Officers

BOCH and Merchants Bank have previously entered into employment agreements with BOCH’s four executive officers, Randall S. Eslick, Robert H. Muttera, Carl W. Rood, and James A. Sundquist. The employment agreements provide for change-in-control, which we refer to as CIC, severance payments in the event that the executive’s employment is terminated by the acquiring or surviving company other than for “cause” or by the executive for “good reason” within one year after consummation of the change in control. It is anticipated that the employment of Messrs. Muttera, Rood, and Sundquist will be terminated upon or shortly after the closing of the merger. As discussed in more detail below, Columbia and Columbia Bank have entered

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into an employment agreement with Mr. Eslick that will become effective upon completion of the merger. Columbia has also agreed that Mr. Eslick will be paid the benefits to which he would have been entitled under his employment agreement as if his employment were terminated following completion of the mergers.

The CIC payments and benefits payable under the above-referenced employment agreements generally include:

Each of the above-referenced employment agreements also includes a one-year customer non-solicitation condition following termination of employment.

In addition to the CIC payments and benefits payable under the employment agreements, BOCH has agreed to pay each of Messrs. Muttera, Rood and Sundquist a lump-sum payment equal to his 2021 target cash incentive bonus under BOCH’s 2021 Short-Term Program, pro-rated through and payable immediately prior to the closing of the merger.

Employment Agreement of Mr. Eslick with Columbia

In connection with the execution of the merger agreement, Mr. Eslick entered into an employment agreement with Columbia and Columbia Bank. The employment agreement will become effective immediately following the effective time of the merger, and provides that Mr. Eslick will serve as President, Merchants Bank of Commerce division for the surviving entity. The employment agreement has an initial term of two years, and may be extended for an additional 12 months upon mutual agreement.

Mr. Eslick will receive an annual (calendar year) base salary of $490,000, and will be eligible to receive a bonus based on a target opportunity of 20% of such base salary, as determined by the Personnel and Compensation Committee of Columbia’s board of directors. In addition, within 60 days following the effective date, Mr. Eslick will receive a one-time award of 6,000 restricted shares of Columbia’s common stock, which shares will vest in full at the end of the initial two-year term, subject to Mr. Eslick’s continued employment with Columbia. Mr. Eslick will also be entitled to receive benefits similar to those offered by Columbia to similarly situated employees of Columbia.

In lieu and in full satisfaction of the potential CIC benefits payable under Mr. Eslick’s employment agreement with BOCH and Merchants Bank, the Columbia employment agreement provides that Columbia will make a lump-sum payment to Mr. Eslick in the amount of $1,872,782 within 15 days following the closing of the merger. The employment agreement further provides that if, during the initial two-year term, Mr. Eslick’s employment is terminated by Columbia without “cause” or by Mr. Eslick for “good reason”, Mr. Eslick will be entitled to receive a lump sum payment equal to the base salary that would have been payable over the remainder of the initial two-year term, and accelerated vesting of a pro rata portion of the one-time award of 6,000 restricted shares.

Under the Columbia employment agreement, Columbia also will assume Mr. Eslick’s existing salary continuation agreement with Merchants Bank. The employment agreement includes a one-year noncompetition covenant covering counties in which Merchants Bank maintains a branch or office or has documented plans to establish a branch or office, and a one-year customer nonsolicitation condition.

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Merger-Related Compensation for BOCH’s Named Executive Officers

The following table and the related footnotes provide information required by Item 402(t) of Regulation S-K about the compensation that may be paid or become payable to BOCH’s named executive officers that is based on or otherwise relates to the mergers. The compensation shown in this table and described in these footnotes is the subject of a non-binding advisory vote of the BOCH shareholders at the BOCH special meeting, as described in “BOCH Proposals—Merger-Related Named Executive Officer Compensation Proposal.” The figures in the table are estimated based on compensation levels as of the date of this proxy statement/prospectus and an assumed effective date of December 31, 2021, for both the merger and, where applicable, termination of the executive officer’s employment. The amounts reported below are estimates based on multiple assumptions that may or may not actually occur or be accurate on the relevant date, including assumptions described in this proxy statement/prospectus, and do not reflect certain compensation actions that may occur before the completion of the merger. All amounts below that are determined using the value per BOCH common share have been calculated based on a price per BOCH common share of $14.98 (the average closing market price of BOCH common shares over the first five business days following the public announcement of the mergers on June 23, 2021, as reported by the Nasdaq Global Market). The amounts set forth below are subject to reduction to the extent necessary to avoid imposition of the excise tax imposed under Section 4999 of the Code on excess parachute payments as defined in Code Section 280G. As a result of the foregoing issues and assumptions, the actual amounts, if any, to be received by an executive officer may differ materially from the amounts set forth below.

Golden Parachute Compensation

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THE MERGER AGREEMENT

Effects of the Mergers

As a result of the parent merger, BOCH will merge with and into Columbia with Columbia continuing as the surviving corporation in the parent merger. Promptly following the parent merger, Merchants Bank will merge with and into Columbia Bank, with Columbia Bank as the surviving bank in the bank merger. As a result of the mergers, there will no longer be any publicly held common shares of BOCH. In receiving Columbia common shares as merger consideration, former BOCH shareholders will only participate in Columbia’s future earnings and potential growth through their ownership of Columbia common shares. All of the other incidents of direct stock ownership in BOCH, such as the right to vote on certain corporate decisions, to elect directors and to receive dividends and distributions from BOCH, will be extinguished upon completion of the parent merger.

Effective Time of the Parent Merger

The closing of the parent merger will occur on the first business day of the first calendar month that follows the month in which the last to be satisfied of the closing conditions is satisfied (other than those conditions that by their nature are to be satisfied or waived at the closing, but subject to the satisfaction or waiver of those conditions), unless the parties mutually agree to extend the closing. However, if the last to be satisfied of the closing conditions is satisfied in the month of November 2021, then the closing date will be January 1, 2022. The parent merger will be completed legally at the date and time specified in the articles of merger to be filed by Columbia with the Secretary of State of the State of California. As of the date of this document, the parties expect that the parent merger will be effective by the end of 2021. However, there can be no assurance as to when or if the parent merger will occur.

As described below, if the parent merger has not closed by March 31, 2022, which date may be extended under certain conditions, the merger agreement may be terminated by either BOCH or Columbia, unless the failure of the closing to occur by such date is due to the failure of the party seeking to terminate the merger agreement to perform or observe the covenants and agreements of such party set forth in the merger agreement.

Covenants and Agreements

Conduct of Businesses Prior to the Completion of the Parent Merger

BOCH has agreed that, except as previously disclosed, as expressly contemplated by or permitted by the merger agreement, as required by applicable law or with the prior written consent of Columbia, prior to the date and time when the parent merger becomes effective, BOCH will, and will cause each of its subsidiaries to, conduct its business, and cause its subsidiaries to conduct their respective businesses, in the ordinary course consistent with past practice in all material respects and use commercially reasonable efforts to maintain and preserve intact its business organization and advantageous business relationships, and goodwill with governmental entities, customers, suppliers, distributors, creditors, lessors, officers and employees and business associates and keep available the services of BOCH and its subsidiaries’ present employees and agents. BOCH and Columbia have agreed to take no action (and to cause their subsidiaries to take no action) that is intended to or would reasonably be expected to adversely affect or materially delay the ability of either BOCH or Columbia to obtain any necessary approvals of any regulatory agency or other governmental entity required for the transactions contemplated by the merger agreement or to perform the covenants and agreements in the merger agreement or to consummate the transactions contemplated by the merger agreement.

In addition to the general covenants above, BOCH has agreed that prior to the effective time of the parent merger, except as previously disclosed, subject to specified exceptions, it will not, and will not permit its subsidiaries to, without the prior written consent of Columbia (which may not be unreasonably withheld):

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Columbia has agreed to a more limited set of restrictions on its business prior to the completion of the parent merger. Specifically, Columbia has agreed that prior to the effective time of the parent merger, except as expressly permitted by the merger agreement, it will not, and will not permit its subsidiaries to, except as may be required by applicable law or policies imposed by any governmental entity, without the prior written consent of BOCH (which will not be unreasonably withheld):

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Regulatory Matters

Columbia and BOCH have agreed to use their commercially reasonable efforts to promptly prepare and file with the SEC within 45 days after the date of the merger agreement, and in any event as soon as reasonably practicable thereafter, a registration statement on Form S-4, in which this proxy statement/prospectus is included. Each of Columbia and BOCH has agreed to use its commercially reasonable efforts to have the Form S-4 declared effective under the Securities Act as promptly as practicable after such filing, and BOCH and Columbia agreed to then mail or deliver the proxy statement/prospectus to their respective shareholders. Columbia also agreed to use its commercially reasonable efforts to obtain all necessary state securities law or “blue sky” permits and approvals required to carry out the transactions contemplated by the merger agreement, and BOCH agreed to furnish all information concerning BOCH and the holders of BOCH common shares as may be reasonably requested in connection with any such action.

Columbia and BOCH have agreed to reasonably cooperate with each other and use their respective commercially reasonable efforts to promptly prepare all necessary documentation to effect all applications, notices, petitions and filings, to obtain as promptly as practicable all permits, consents, approvals and authorizations of all third parties and governmental entities that are necessary or advisable to consummate the mergers and the other transactions contemplated by the merger agreement as soon as reasonably practicable, and to comply with the terms and conditions of all such permits, consents, approvals and authorizations of all such third parties or governmental entities. Columbia agreed to use its commercially reasonable efforts to make all initial requisite regulatory filings within 20 business days of the date of the merger agreement, and in any event no later than 30 days following the date of the merger agreement (other than any notice to the Federal Reserve under its regulations, which will be filed in accordance with the timing contemplated by such regulations). BOCH and Columbia have the right to review in advance and, to the extent practicable, each will consult the other on, in each case subject to applicable laws, all the non-confidential information relating to BOCH or Columbia (excluding any confidential financial information relating to individuals), as the case may be, and any of their respective subsidiaries, that appear in any filing made with, or written materials submitted to, any third party or any governmental entity in connection with the transactions contemplated by the merger agreement. In exercising this right, each of the parties has agreed to act reasonably and as promptly as practicable. BOCH and Columbia will consult with each other with respect to the obtaining of all permits, consents, approvals and authorizations of all third parties and governmental entities necessary or advisable to consummate the mergers and the other transactions contemplated by the merger agreement and each party will keep the other reasonably apprised of the status of matters relating to such approvals and the completion of the mergers and the other transactions contemplated by the merger agreement. Each party will consult with the other in advance of any meeting or conference with any governmental entity in connection with the mergers and the other transactions contemplated by the merger agreement.

Each of Columbia and BOCH has agreed to furnish to the other, upon request, all information concerning itself, its subsidiaries, directors, officers and shareholders and such other matters as may be reasonably necessary or advisable in connection with this proxy statement/prospectus, the Form S-4 or any other statement, filing, notice or application made by or on behalf of Columbia, BOCH or any of their respective subsidiaries to any governmental entity in connection with the mergers and the other transactions contemplated by the merger agreement.

Each of Columbia and BOCH has agreed to cooperate with the other and use its reasonable best efforts in good faith to take, or cause to be taken, all actions, and to do, or cause to be done, all things necessary, proper or desirable, or advisable on its part under the merger agreement or under applicable laws to consummate and make effective the mergers, and the other transactions contemplated as promptly as practicable, including the satisfaction of the closing conditions set forth in the merger agreement.

Neither Columbia nor any of its subsidiaries are required to take any action, or commit to take any action, or agree to any condition or restriction, in connection with obtaining any permits, consents, approvals and authorizations of any governmental entities that would reasonably be likely, in each case following the effective

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time of the parent merger (but regardless when the action, condition or restriction is taken or implemented), to have a material adverse effect on Columbia (measured on a scale relative to BOCH), a material adverse effect on BOCH or materially restrict or impose a material burden on Columbia or any of its subsidiaries (including, after the effective time of the parent merger, BOCH and its subsidiaries) in connection with the transactions contemplated by the merger agreement or with respect to the business or operation of Columbia or any of its subsidiaries (including, after the effective time of the parent merger, BOCH and its subsidiaries), which we refer to as a materially burdensome regulatory condition.

Each of Columbia and BOCH will promptly advise the other upon receiving any communication from any governmental entity the consent or approval of which is required for consummation of the mergers and the other transactions contemplated by the merger agreement that causes such party to believe that there is a reasonable likelihood that any requisite regulatory approval will not be obtained or that the receipt of any such approval may be materially delayed.

Shareholder Approval

BOCH has agreed to convene a special meeting of its shareholders as soon as practicable after the Form S-4 is declared effective to consider and vote upon the proposals necessary to consummate the mergers, and BOCH has agreed to recommend to its shareholders that they approve such proposals. BOCH may change its recommendation to its shareholders if, following receipt of a superior proposal (as defined below), the BOCH board of directors concludes in good faith (after consultation with outside legal counsel) that the failure to take such action would reasonably be expected to result in a violation of its fiduciary duties under applicable law.

Nasdaq Listing

Columbia has agreed to file with the Nasdaq Global Select Market any required notices or forms with respect to the Columbia common shares to be issued in the parent merger.

Employee Matters

The merger agreement provides that for the period beginning on the effective time of the parent merger and ending on the 1-year anniversary of the effective time of the parent merger, Columbia will provide each employee who is actively employed by BOCH or its subsidiaries on the closing date, while employed by Columbia or any of its subsidiaries following the effective time of the parent merger, whom we refer to as a continuing employee, with: (i) base salary no less favorable than the base salary provided to such continuing employees immediately prior to the effective time of the parent merger; (ii) annual cash bonus opportunities no less favorable than annual cash bonus opportunities provided by Columbia to similarly situated employees of Columbia; (iii) employee benefits which, in the aggregate, are no less favorable than employee benefits provided by Columbia to similarly situated employees of Columbia and (iv) certain agreed severance terms, provided that until Columbia causes such continuing employees to participate in Columbia employee benefit plans, a continuing employee’s continued participation in a BOCH employee benefit plan satisfies this provision.

Columbia will use commercially reasonable efforts to (i) cause any pre-existing conditions or limitations and eligibility waiting periods under any group health plans of Columbia or its affiliates to be waived with respect to the continuing employees and their eligible dependents, (ii) give each continuing employee credit for the plan year in which the effective time of the parent merger occurs towards applicable deductibles and annual out-of-pocket limits for medical expenses incurred prior to the effective time of the merger for which payment has been made and (iii) give each continuing employee service credit for such continuing employee’s employment with BOCH and its subsidiaries for purposes of vesting, benefit accrual and eligibility to participate under each applicable Columbia benefit plan, as if such service had been performed with Columbia, except for benefit accrual under defined benefit pension plans, for purposes of qualifying for subsidized early retirement benefits or to the extent it would result in a duplication of benefits.

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If requested in writing by Columbia at least 15 calendar days prior to the effective time of the parent merger, BOCH will take (or cause to be taken) all actions reasonably determined by Columbia to be necessary or appropriate to terminate, effective not later than the business day immediately prior to the effective time of the parent merger, any BOCH employee benefit plan that contains a cash or deferred arrangement intended to qualify under Section 401(k) of the Code, provided that such actions are in compliance with applicable law. In the event that Columbia requests that such plan(s) be terminated, BOCH will provide Columbia with evidence that such plan(s) has been terminated (the form and substance of which will be subject to review and approval by Columbia, approval of which will not be unreasonably withheld) not later than the business day immediately preceding the effective time of the parent merger.

From and after the date of the merger agreement, prior to making any written or oral communications to officers or employees of BOCH or any of its subsidiaries pertaining to compensation, benefit or other employment-related matters that are affected by the transactions contemplated by the merger agreement, BOCH will provide Columbia with a copy of the intended communication or talking points, Columbia will have a reasonable period of time to review and comment on the communication, and Columbia and BOCH will cooperate in providing any such mutually agreeable communication.

Columbia has agreed to satisfy all amounts duly owed to officers and directors of BOCH and its subsidiaries pursuant to BOCH’s salary continuation agreement and directors deferred compensation plans, as in effect as of the date of the merger agreement, in accordance with the provisions or elections as to timing specified therein.

Indemnification; Directors’ and Officers’ Insurance

From and after the effective time of the parent merger, Columbia will indemnify and hold harmless each present and former director and officer of BOCH and its subsidiaries (in each case, when acting in such capacity) against any costs or expenses (including reasonable, documented attorneys’ fees), judgments, fines, losses, claims, damages or liabilities incurred in connection with any claim, action, suit, proceeding or investigation, whether civil, criminal, administrative or investigative, arising out of actions or omissions occurring at or prior to the effective time of the parent merger, including the transactions contemplated by the merger agreement, to the extent they are indemnified by BOCH or its subsidiaries on the date of the merger agreement, to the fullest extent permitted under applicable law. Columbia has also agreed to advance expenses as incurred to the fullest extent permitted under applicable law; provided that the indemnified party to whom expenses are advanced provides an undertaking to repay such advances if it is ultimately determined that such indemnified party is not entitled to indemnification.

For a period of six years following the effective time of the parent merger, Columbia will provide director’s and officer’s liability insurance that serves to reimburse the present and former officers and directors of BOCH or any of its subsidiaries as of the effective time of the parent merger (providing only for the Side A coverage where the existing policies also include Side B coverage for BOCH) with respect to claims against such directors and officers arising from facts or events occurring before the effective time of the parent merger (including the transactions contemplated by the merger agreement), which insurance will contain at least the same coverage and amounts, and contain terms and conditions no less advantageous to such persons as that coverage currently provided by BOCH, except that Columbia is not required to expend in the aggregate for such six-year period, an amount in excess of 250% of the aggregate annual premiums paid as of the date of the merger agreement by BOCH for any such insurance, provided that, if Columbia is unable to maintain or obtain such director’s and officer’s liability insurance, Columbia shall obtain as much comparable insurance as is available at a cost in the aggregate for such 6-year period up to 250% of the current annual premium, and provided further that, officers and directors of BOCH may be required to make applications and provide customary representations and warranties to Columbia’s or the surviving corporation’s insurance carrier for the purpose of obtaining such insurance. Prior to the effective time of the parent merger, and in lieu of the foregoing, BOCH will use reasonable best efforts to purchase a six-year prepaid tail policy for directors’ and officers’ liability insurance on the terms described above, and fully pay for such policy prior to the effective time of the parent merger, at an aggregate cost up to, but not exceeding 250% of the current annual premium for such insurance.

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No Solicitation; Change in Board Recommendation

The merger agreement provides that neither BOCH, its subsidiaries nor any of their respective officers, directors and employees will, and will cause its and its subsidiaries’ officers, directors, agents, representatives, advisors and affiliates not to, initiate, solicit, encourage or knowingly facilitate any inquiries or the making of proposals with respect to, or engage in any negotiations concerning, or provide any confidential or nonpublic information or data to, or have any discussions with, any person relating to, any acquisition proposal (as defined below) or otherwise facilitate any effort to attempt or make or implement an acquisition proposal. However, if at any time after the date of the merger agreement and prior to, but not after, obtaining the approval of the merger agreement by BOCH shareholders, BOCH receives an unsolicited bona fide acquisition proposal and the BOCH board of directors concludes in good faith that such acquisition proposal constitutes, or is reasonably expected to result in, a superior proposal (as defined below), then BOCH and its board of directors may, and may permit its subsidiaries and BOCH’s and its subsidiaries’ representatives to, furnish or cause to be furnished nonpublic information and participate in such negotiations or discussions to the extent that the BOCH board of directors concludes in good faith (after consultation with outside legal counsel) that failure to take such actions would reasonably be expected to result in a violation of its fiduciary duties under applicable law; provided that prior to providing any such nonpublic information or engaging in any such negotiations, BOCH must have entered into a confidentiality agreement with such third party on terms no less restrictive in the aggregate to the counterparty than those contained in the confidentiality agreement between BOCH and Columbia, and which expressly permits BOCH to comply with its obligations pursuant to the merger agreement. BOCH will immediately cease and cause to be terminated any activities, discussions or negotiations conducted on or before the date of the merger agreement with any persons other than Columbia with respect to any acquisition proposal and will use its reasonable best efforts, subject to applicable law, to (i) enforce any confidentiality or similar agreement relating to an acquisition proposal and (ii) within 10 business days after the date of the merger agreement, request and confirm the return or destruction of any confidential information provided to any person (other than Columbia and its affiliates) pursuant to any such confidentiality or similar agreement. BOCH must promptly (and in any event within 24 hours) advise Columbia following receipt of any acquisition proposal, of any discussions or negotiations that are sought to be initiated or continued or any request for nonpublic information or inquiry that would reasonably be expected to lead to any acquisition proposal and the substance thereof (including the identity of the person making such acquisition proposal), and keep Columbia promptly apprised of any related developments, discussions and negotiations (including the terms and conditions of any such request, inquiry or acquisition proposal, or all amendments or proposed amendments thereto) on a current basis (it being understood that no such communications to Columbia will be deemed an adverse change of recommendation (as defined below)). BOCH agrees that it will contemporaneously provide to Columbia any confidential or nonpublic information concerning BOCH or any of its subsidiaries that may be provided to any other person in connection with any acquisition proposal which has not previously been provided to Columbia.

The term “acquisition proposal” means a tender or exchange offer, proposal for a merger, consolidation, sale of assets or other business combination involving BOCH or any of its subsidiaries or any proposal or offer to acquire in any manner more than 15% of the voting power in, or more than 15% of the fair market value of the business, assets or deposits of, BOCH or any of its subsidiaries or any public announcement of a proposed plan or intention to do any of the foregoing or any agreements to engage in any of the foregoing, other than the transactions contemplated by the merger agreement and any sale of whole loans and securitizations in the ordinary course. The term “superior proposal” means an unsolicited bona fide written acquisition proposal that the BOCH board of directors concludes in good faith to be more favorable from a financial point of view to its shareholders than the parent merger and the other transactions contemplated by the merger agreement and to be reasonably capable of being consummated on the terms proposed, (i) after receiving the advice of its financial advisors (who must be a nationally recognized investment banking or financial advisory firm), (ii) after taking into account the likelihood of consummation of such transaction on the terms set forth therein and (iii) after taking into account all legal (with the advice of counsel), financial (including the financing terms of any such proposal), regulatory and other aspects of such proposal (including any expense reimbursement provisions and conditions to closing) and any other relevant factors permitted under applicable law, and after taking into account

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any amendment or modification to the merger agreement agreed to by Columbia; provided that for purposes of the definition of superior proposal, the references to “more than 15%” in the definition of acquisition proposal will be deemed to be references to “at least 50%.”

None of the members of the BOCH board of directors may, except as expressly permitted by the merger agreement, withdraw or materially and adversely modify his or her recommendation that BOCH shareholders vote to approve the principal terms of the merger agreement, or recommend to BOCH shareholders an acquisition proposal other than the parent merger, which we refer to as an adverse change of recommendation, or cause or commit BOCH to enter into any agreement or understanding other than the confidentiality agreement referred to above relating to any acquisition proposal made to BOCH. Nevertheless, in the event that BOCH receives an acquisition proposal that the BOCH board of directors concludes in good faith constitutes a superior proposal, the BOCH board of directors may make an adverse change of recommendation or terminate the merger agreement, if it concludes in good faith (after consultation with outside legal counsel) that failure to take such actions would reasonably be expected to result in a violation of its fiduciary duties under applicable law, as long as BOCH gives Columbia prior written notice at least 5 business days before taking such action and during such 5-business day period BOCH negotiates in good faith with Columbia to enable Columbia to make an improved offer that is at least as favorable to the shareholders of BOCH as such alternative acquisition proposal.

Representations and Warranties

The merger agreement contains representations and warranties made by BOCH to Columbia relating to a number of matters, including the following:

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The merger agreement also contains representations and warranties made by Columbia to BOCH relating to a number of matters, including the following:

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Certain of these representations and warranties are qualified as to “materiality” or “material adverse effect”. For purposes of the merger agreement, a “material adverse effect” with respect to BOCH or Columbia, as the case may be, means, with respect to any party, a material adverse effect on (a) the business, assets or deposit liabilities, properties, operations, condition (financial or otherwise) or results of operations of such party and its subsidiaries taken as a whole or (b) the ability of such party to consummate the transactions contemplated by the merger agreement on a timely basis and in any event on or before March 31, 2022; provided, however, that, with respect to clause (a) only, a material adverse effect will not be deemed to include effects to the extent arising out of, relating to or resulting from (A) changes after the date of the merger agreement in applicable GAAP or regulatory accounting requirements generally affecting other companies in the banking industries in which such party and its subsidiaries operate, (B) changes after the date of the merger agreement in laws, rules or regulations of general applicability to companies of similar size in the banking industries in which such party and its subsidiaries operate or interpretations thereof, (C) changes after the date of the merger agreement in global, national or regional political conditions or general economic or market conditions (including changes in prevailing interest rates, credit availability and liquidity, currency exchange rates, and price levels or trading volumes in the United States or foreign securities markets) affecting financial institutions generally, (D) changes after the date of the merger agreement in the credit markets, any downgrades in the credit markets, or adverse credit events resulting in deterioration in the credit markets generally and not specifically relating to such party or its subsidiaries, (E) a decline in the trading price of a party’s common shares or a failure, in and of itself, to meet earnings projections, but not, in either case, including any underlying causes thereof, (F) the entry into or announcement of the merger agreement or the transactions contemplated by the merger agreement or the consummation of the transactions contemplated by the merger agreement, (G) any outbreak or escalation of hostilities, declared or undeclared acts of war or terrorism or (H) actions or omissions taken with the prior written consent of the other party or expressly required by the merger agreement; provided, further, that any effect attributable to or resulting from any of the changes, events, conditions or trends described in clauses (A), (B), (C), (D), (E) and (G) may constitute, and may be taken into account in determining the occurrence of a material adverse effect if and only to the extent they have a disproportionate adverse impact on such party and its subsidiaries, taken as a whole, as compared to other companies of similar size in the banking industry in which such party and its subsidiaries operate.

The representations and warranties in the merger agreement do not survive the effective time of the parent merger and, as described below under “Effect of Termination,” if the merger agreement is validly terminated, there will be no liability under the representations and warranties of the parties, unless a party knowingly breached the merger agreement.

This summary and the copy of the merger agreement attached to this document as Appendix A are included solely to provide investors with information regarding the terms of the merger agreement. They are not intended to provide factual information about the parties or any of their respective subsidiaries or affiliates. The merger agreement contains representations and warranties by Columbia and BOCH, which were made only for purposes of that agreement and as of specific dates. The representations, warranties and covenants in the merger agreement were made solely for the benefit of the parties to the merger agreement, may be subject to limitations agreed upon by the contracting parties, including being qualified by confidential disclosures made for the purposes of allocating contractual risk between the parties to the merger agreement instead of establishing these matters as facts and may be subject to standards of materiality applicable to the contracting parties that differ from those generally applicable to investors. In reviewing the representations, warranties and covenants contained in the merger agreement or any descriptions thereof in this summary, it is important to bear in mind that such representations, warranties and covenants or any descriptions thereof were not intended by the parties to the merger agreement to be characterizations of the actual state of facts or condition of Columbia, BOCH or any of their respective subsidiaries or affiliates. Moreover, information concerning the subject matter of the representations, warranties and covenants may change after the date of the merger agreement, which subsequent information may or may not be fully reflected in Columbia’s and BOCH’s public disclosures. For the foregoing reasons, the representations, warranties and covenants or any descriptions of those provisions should not be read alone and should instead be read in conjunction with the other information contained in the reports, statements

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and filings that Columbia and BOCH publicly file with the SEC. For more information regarding these documents, see the section entitled “Where You Can Find More Information” included elsewhere in this proxy statement/prospectus.

Conditions to the Parent Merger

Conditions to Each Party’s Obligations

The respective obligations of each of Columbia and BOCH to complete the parent merger are subject to the satisfaction of the following conditions:

Conditions to Obligations of Columbia

The obligation of Columbia to complete the parent merger is also subject to the satisfaction, or waiver by Columbia, at or prior to the effective time of the parent merger, of the following conditions:

Conditions to Obligations of BOCH

The obligation of BOCH to complete the merger is also subject to the satisfaction, or waiver by BOCH, at or prior to the effective time of the merger, of the following conditions:

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Termination; Termination Fee

The merger agreement may be terminated at any time prior to the effective time of the parent merger, whether before or after approval of matters presented in connection with the parent merger to the shareholders of Columbia and BOCH:

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BOCH must pay Columbia a termination fee of $12,000,000 in the event that:

Effect of Termination

If the merger agreement is validly terminated, the merger agreement will become void and have no effect, and none of BOCH, Columbia, any of their respective subsidiaries or any of the officers or directors of any of them will have any liability of any nature whatsoever under the merger agreement, or in connection with the transactions contemplated by the merger agreement, except that (i) the provisions of the merger agreement relating to confidentiality obligations of the parties, the termination fees, publicity and certain other technical provisions will continue in effect notwithstanding termination of the merger agreement and (ii) neither BOCH nor Columbia will be relieved or released from any liabilities or damages arising out of its knowing breach of the merger agreement (which, in the case of BOCH includes the loss to BOCH’s shareholders of the economic benefits of the parent merger).

Amendments, Extensions and Waivers

The merger agreement may be amended by the parties, by action taken or authorized by their respective boards of directors, at any time before or after approval of the matters presented in connection with the parent merger by the shareholders of BOCH or Columbia, in writing signed on behalf of each of the parties, provided

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that after any approval of the transactions contemplated by the merger agreement by such shareholders, there may not be, without further approval of such shareholders, any amendment of the merger agreement that requires further approval under applicable law.

At any time prior to the effective time of the parent merger, the parties, by action taken or authorized by their respective boards of directors, may, to the extent legally allowed, (a) extend the time for the performance of any of the obligations or other acts of the other party, (b) waive any inaccuracies in the representations and warranties contained in the merger agreement or (c) waive compliance with any of the agreements or conditions contained in the merger agreement. Any agreement on the part of a party to any such extension or waiver must be in writing signed on behalf of such party. Any such extension or waiver or failure to insist on strict compliance with an obligation, covenant, agreement or condition will not operate as a waiver of, or estoppel with respect to, any subsequent or other failure.

Fees and Expenses

Except for the termination fee, as described elsewhere in this document, all fees and expenses incurred in connection with the mergers, the merger agreement and the transactions contemplated by the merger agreement (including costs and expenses of printing and mailing this document) will be paid by the party incurring such fees or expenses, whether or not the parent merger is completed.

Related Agreements

Each member of the BOCH board of directors has entered into a voting support agreement with Columbia and BOCH pursuant to which such directors have agreed, until the earlier of the closing or the termination of the merger agreement in accordance with its terms, to vote his or her BOCH common shares in favor of approval of (i) the principal terms of the merger agreement and the transactions contemplated thereby, (ii) any other matter that is required to facilitate the transactions contemplated by the merger agreement and (iii) any proposal to adjourn or postpone the BOCH special meeting to a later date if there are not sufficient votes to approve the principal terms of the merger agreement. Such directors have also agreed to vote against any action or agreement submitted for approval to the shareholders of BOCH that would (i) result in a breach of any covenant, representation or warranty or any other obligation or agreement of BOCH under the merger agreement, (ii) result in any of the conditions to the consummation of the parent merger under the merger agreement not being fulfilled, (iii) be in competition with or opposition to the merger agreement or the parent merger, (iv) be an acquisition proposal or (v) impair the ability of Columbia to complete the parent merger, the ability of BOCH to complete the parent merger, or that would otherwise be inconsistent with, prevent, impede or delay the consummation of the transactions contemplated by the merger agreement. The voting support agreements apply solely to the directors in their capacities as shareholders of BOCH and do not prevent them from discharging their fiduciary duties with respect to their roles on the BOCH board of directors. As of April 6, 2021, the directors who are parties to the voting support agreements have the right to vote, in the aggregate, 833,185 outstanding BOCH common shares, which represents approximately 4.98% of the outstanding BOCH common shares entitled to vote.

The voting support agreements also provide that the directors will not transfer (other than for estate planning or philanthropic purposes) the BOCH common shares that they own until the earlier of the closing or the termination of the merger agreement in accordance with its terms.

The voting support agreements also prohibit the directors for a two-year period following the closing from (a) soliciting or attempting to solicit (i) any employees or independent contractors of Columbia, BOCH or the combined company to participate in a competing business, (ii) any customers, business partners or joint venturers of Columbia, BOCH or the combined company to transfer their business to a competing business or to reduce their business or cease conducting business with Columbia, BOCH or the combined company or (iii) the termination of an employment or contractual relationship between Columbia, BOCH or the combined company

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and any employee, independent contractor, customer, business partner or joint venturer, (b) hiring any employees who were then employed by the combined company or who were employed by Columbia or BOCH prior to the effective time of the parent merger, subject to certain exceptions or (c) in any other way interfering with or disrupting Columbia, BOCH or the combined company’s relationship with any of its employees, independent contractors, customers, business partners or joint venturers.

Pursuant to the voting support agreements, BOCH directors party thereto have agreed to tender their resignations from the BOCH board of directors, subject to and effective upon the closing. The voting support agreements terminate automatically in the event that the merger agreement is terminated in accordance with its terms.

The form of voting support agreement is attached to this proxy statement/prospectus as Appendix B.

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MATERIAL U.S. FEDERAL INCOME TAX CONSEQUENCES OF THE PARENT MERGER

This discussion addresses the material U.S. federal income tax consequences of the parent merger to U.S. holders (as defined below) of BOCH common shares. The discussion is based on the provisions of the Code, U.S. Treasury regulations, administrative rulings and judicial decisions, all as currently in effect and all of which are subject to change (possibly with retroactive effect) and to differing interpretations.

For purposes of this discussion, we use the term “U.S. holder” to mean:

This discussion applies only to BOCH shareholders that hold their BOCH common shares as a capital asset within the meaning of Section 1221 of the Code (generally, property held for investment), and does not address all aspects of U.S. federal income taxation that may be relevant to a particular U.S. holder in light of its personal circumstances or to U.S. holders subject to special treatment under the U.S. federal income tax laws, including:

If a partnership or other entity taxed as a partnership holds BOCH common shares, the tax treatment of a partner in the partnership will depend upon the status of the partner and the activities of the partnership. Partnerships and partners in such a partnership should consult their tax advisors about the tax consequences of the parent merger to them.

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The actual tax consequences of the parent merger to you may be complex and will depend on your specific situation and on factors that are not within our control. You should consult with your own tax advisor as to the tax consequences of the parent merger in your particular circumstances, including the applicability and effect of the alternative minimum tax, the unearned income Medicare contribution tax and any state, local or foreign and other tax laws and of changes in those laws.

U.S. Federal Income Tax Consequences of the Parent Merger

The parties intend for the parent merger to qualify as a “reorganization” within the meaning of Section 368(a) of the Code. It is a condition to BOCH’s obligation to complete the parent merger that BOCH receives a written opinion of its counsel, Miller Nash, dated as of the closing date, to the effect that the parent merger will qualify as a reorganization within the meaning of Section 368(a) of the Code. It is a condition to Columbia’s obligation to complete the parent merger that Columbia receives an opinion of its counsel, Sullivan & Cromwell, dated as of the closing date, to the effect that the parent merger will qualify as a reorganization within the meaning of Section 368(a) of the Code. These opinions will be based on the assumption that the parent merger will be completed in the manner set forth in the merger agreement and the registration statement on Form S-4 of which this proxy statement/prospectus forms a part, and on representation letters provided by Columbia and BOCH to be delivered at the time of the closing. Those opinions will also be based on the assumption that the representations found in the representation letters are, as of the effective time of the parent merger, true and complete without qualification and that the representation letters are executed by appropriate and authorized officers of Columbia and BOCH. If any of the assumptions or representations upon which such opinions are based is inconsistent with the actual facts with respect to the parent merger the U.S. federal income tax consequences of the parent merger could be adversely affected.

In addition, neither of the tax opinions given in connection with the parent merger or in connection with the filing of this registration statement will be binding on the Internal Revenue Service. Neither Columbia nor BOCH intends to request any ruling from the Internal Revenue Service as to the U.S. federal income tax consequences of the parent merger, and consequently, there is no guarantee that the Internal Revenue Service will treat the parent merger as a “reorganization” within the meaning of Section 368(a) of the Code.

Based solely on the information, and subject to the assumptions, qualifications and limitations set forth herein and in their respective federal income tax opinions filed herewith, this discussion of the material U.S. federal income tax consequences of the parent merger, to the extent such discussion expresses conclusions as to the application of U.S. federal income tax law, constitutes the opinions of Sullivan & Cromwell, counsel to Columbia, and Miller Nash, counsel to BOCH.

As a “reorganization,” the material U.S. federal income tax consequences of the parent merger to U.S. holders of BOCH common shares will be as follows:

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If a U.S. holder of BOCH common shares acquired different blocks of BOCH common shares at different times or at different prices, such holder’s basis and holding period may be determined with reference to each block of BOCH common shares. Any such holder should consult its tax advisors regarding the manner in which Columbia common shares received in the exchange should be allocated among different blocks of BOCH common shares and with respect to identifying the bases or holding periods of the particular Columbia common shares received in the parent merger.

Cash Received In Lieu of a Fractional Columbia Common Share

A U.S. holder of BOCH common shares who receives cash in lieu of a fractional Columbia common share will be treated as having received the fractional share pursuant to the parent merger and then as having exchanged the fractional share for cash in a redemption by Columbia. As a result, a BOCH shareholder will generally recognize gain or loss equal to the difference between the amount of cash received and the basis in his or her fractional share interest as set forth above. This gain or loss will generally be capital gain or loss, and will be long-term capital gain or loss if, as of the effective date of the parent merger, the holding period for such shares is greater than one year. The deductibility of capital losses is subject to limitations.

Information Reporting and Backup Withholding

A non-corporate U.S. holder may be subject to information reporting and backup withholding (currently at a rate of 24%) on any cash received in the parent merger, including cash received in lieu of a fractional Columbia common share. Backup withholding generally will not apply, however, to such U.S. holders who:

Any amounts withheld under the backup withholding rules are not additional tax and will generally be allowed as a refund or credit against your U.S. federal income tax liability, provided the required information is timely furnished to the Internal Revenue Service.

The preceding discussion is intended only as a summary of material U.S. federal income tax consequences of the parent merger. It is not a complete analysis or discussion of all potential tax effects that may be important to you. Thus, you are strongly encouraged to consult your tax advisor as to the specific tax consequences resulting from the parent merger, including tax return reporting requirements, the applicability and effect of federal, state, local, and other tax laws and the effect of any proposed changes in the tax laws.

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DESCRIPTION OF COLUMBIA’S CAPITAL STOCK

Columbia’s authorized capital stock consists of 115,000,000 common shares, no par value per share, and 2,000,000 preferred shares, no par value per share.

Common Shares

General

The holders of Columbia common shares have one vote per share on all matters submitted to a vote of Columbia’s shareholders. There are no cumulative voting rights for the election of directors. Holders of common shares are entitled to receive ratably such dividends as may be declared by the board of directors out of legally available funds, subject to preferences that may be applicable to any outstanding series of preferred stock. In the event of a liquidation, dissolution or winding up of Columbia, the holders of common shares are entitled to share ratably in all assets remaining after payment of liabilities and the liquidation preference of any outstanding preferred shares. Holders of Columbia common shares have no preemptive, subscription, redemption, sinking fund or conversion rights.

Dividends

The holders of Columbia common shares are entitled to receive dividends declared by Columbia’s board of directors out of funds legally available therefor. Holders of preferred shares and debt securities, however, have a priority right to distributions and payment over Columbia common shares. Columbia’s ability to pay dividends basically depends on the amount of dividends paid by Columbia’s subsidiaries. The payment of dividends is subject to government regulation, in that regulatory authorities may prohibit banks and bank holding companies from paying dividends in a manner that would constitute an unsafe or unsound banking practice. In addition, a bank may not pay cash dividends if doing so would reduce the amount of its capital below that necessary to meet minimum regulatory capital requirements. State laws also limit a bank’s ability to pay dividends. Accordingly, the dividend restrictions imposed on Columbia’s subsidiaries by statute or regulation effectively may limit the amount of dividends Columbia can pay.

Columbia common shares are listed for trading on the Nasdaq Global Select Market under the symbol “COLB.”

For additional information concerning Columbia’s common shares, see “Comparison of Certain Rights of Holders of Columbia and BOCH Common Shares” below.

Preferred Shares

Under the Columbia articles, Columbia’s board of directors has the authority, without any further vote or action by Columbia’s shareholders, to issue 2,000,000 preferred shares.

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COMPARISON OF CERTAIN RIGHTS OF HOLDERS OF COLUMBIA AND BOCH COMMON SHARES

General

BOCH is incorporated under the laws of the State of California and the rights of BOCH shareholders are governed by the laws of the State of California, the BOCH articles and the BOCH bylaws. As a result of the parent merger, BOCH shareholders will receive Columbia common shares and will become Columbia shareholders. Columbia is incorporated under the laws of the State of Washington and the rights of Columbia shareholders are governed by the laws of the State of Washington, the Columbia articles and the Columbia bylaws. Thus, following the parent merger, the rights of BOCH shareholders who become Columbia shareholders in the parent merger will no longer be governed by the laws of the State of California, the BOCH articles and the BOCH bylaws and instead will be governed by the laws of the State of Washington, as well as by the Columbia articles and the Columbia bylaws.

Comparison of Shareholders’ Rights

Set forth below is a summary comparison of material differences between the rights of Columbia shareholders under the Columbia articles, the Columbia bylaws and Washington law (right column), and the rights of BOCH shareholders under the BOCH articles, BOCH bylaws and California law (left column). The summary set forth below is not intended to provide a comprehensive discussion of each company’s governing documents. This summary is qualified in its entirety by reference to the full text of the Columbia articles and Columbia bylaws, the BOCH articles and BOCH bylaws, the California General Corporation Law, which we refer to as the CGCL, and the Washington Business Corporation Act, which we refer to as the WBCA.

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INFORMATION CONCERNING COLUMBIA

Headquartered in Tacoma, Washington, Columbia is the holding company of Columbia Bank, a Washington state-chartered full service commercial bank. As of August 6, 2021, Columbia had 145 banking offices, including 70 branches in Washington State, 60 branches in Oregon and 15 branches in Idaho. As of June 30, 2021, Columbia had total assets of approximately $18.01 billion, total net loans receivable and loans held for sale of approximately $9.56 billion, total deposits of approximately $15.35 billion and approximately $2.33 billion in shareholders’ equity.

Columbia’s principal office is located at 1301 “A” Street, Tacoma, Washington 98402, and its telephone number at that location is (253) 305-1900. Columbia’s internet address is www.Columbiabank.com. The information provided on the Columbia website is not part of this proxy statement/prospectus and is not incorporated herein by reference. Additional information about Columbia is included in documents incorporated by reference in this document. See “Where You Can Find More Information” and “Documents Incorporated by Reference.”

Columbia’s goal is to be a leading Western regional community banking company while consistently increasing earnings and shareholder value. Its business strategy is to provide customers with the financial sophistication and product depth of a regional banking company while retaining the appeal and service level of a community bank. Columbia continually evaluates its existing business processes while focusing on maintaining asset quality and balanced loan and deposit portfolios, building a strong core deposit base, expanding total revenue and controlling expenses in an effort to increase its return on average equity and gain operational efficiencies. Columbia believes that, as a result of its strong commitment to highly personalized, relationship-oriented customer service, its varied products, its strategic branch locations and the long-standing community presence of its managers, banking officers and branch personnel, it is well positioned to attract and retain new customers and to increase its market share of loans, deposits, investments and other financial services. Columbia is committed to increasing market share in the communities it serves by continuing to leverage its existing branch network, adding new branch locations and considering business combinations that are consistent with its expansion strategy throughout the West.

Columbia’s common shares are traded on the Nasdaq Global Select Market under the symbol “COLB”. Financial and other information relating to Columbia is set forth in its Annual Report on Form 10-K for the year ended December 31, 2020 and in its Quarterly Report on Form 10-Q for the quarter ended June 30, 2021. Information regarding the names, ages, positions, and business backgrounds of the executive officers and directors of Columbia, as well as additional information, including executive compensation, and certain relationships and related person transactions, is set forth in or incorporated by reference in Columbia’s 10-K and in its proxy statement for its 2021 annual meeting of shareholders. See “Documents Incorporated by Reference.”

INFORMATION CONCERNING BOCH

Headquartered in Sacramento, California, BOCH is the holding company of Merchants Bank, a California state-chartered commercial bank, with deposits insured by the FDIC. At August 6, 2021, Merchants Bank had ten full service facilities, one internet banking branch, one limited service facility, and one loan production office in northern California. Merchants Bank provides a wide range of financial services and products for business and retail customers that are competitive with those traditionally offered by banks of similar size in California. At June 30, 2021, BOCH had $1.92 billion in total assets, $1.07 billion in net loans, and $1.70 billion in total deposits.

BOCH common shares are traded on the Nasdaq Global Market under the symbol “BOCH.” Financial and other information relating to BOCH is set forth in its Annual Report on Form 10-K for the year ended December 31, 2020 and its Quarterly Report on Form 10-Q for the quarter ended June 30, 2021. Information

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regarding the names, ages, positions, and business backgrounds of the executive officers and directors of BOCH, as well as additional information, including executive compensation, and certain relationships and related person transactions, is included in BOCH’s Definitive Proxy Statement on Schedule 14A for BOCH’s 2021 Annual Meeting of Shareholders.

BOCH’s principal office is located at 555 Capitol Mall, Suite 1255, Sacramento, California 95814, and its telephone number at that location is (800) 421-2575. BOCH’s internet address is www.bankofcommerceholdings.com. The information provided on the BOCH website is not part of this proxy statement/prospectus and is not incorporated herein by reference.

Additional information about BOCH is included in documents incorporated by reference into this proxy statement/prospectus. See “Where You Can Find More Information” and “Documents Incorporated by Reference” included elsewhere in this proxy statement/prospectus.

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BOCH SPECIAL MEETING OF SHAREHOLDERS

Date, Time and Place

The BOCH board of directors is using this proxy statement/prospectus to solicit proxies from the holders of BOCH common shares for use at the BOCH special meeting of shareholders.

Together with this proxy statement/prospectus, BOCH is also sending you a notice of the BOCH special meeting and a form of proxy that is solicited by the BOCH board of directors. The BOCH special meeting will be held at 1015 7th Street, Sacramento, California, at 4:00 p.m., Pacific Time, on September 21, 2021. On or about August 18, 2021, BOCH commenced mailing this proxy statement/prospectus and the enclosed form of proxy to its shareholders entitled to vote at the BOCH special meeting.

Purpose

At the BOCH special meeting, BOCH shareholders will be asked to:

Solicitation of Proxies

The expense of printing and mailing proxy materials for the BOCH special meeting will be borne by BOCH. In addition to the solicitation of proxies by mail, certain directors, officers and other employees of BOCH may solicit proxies in person, by telephone, or by email. No additional compensation will be paid to such persons for such solicitations. BOCH will reimburse brokers and others for their reasonable expenses in forwarding solicitation material to beneficial owners of BOCH common shares.

Additional Information

For additional information concerning the BOCH special meeting of shareholders, see “Questions and Answers” above.

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BOCH PROPOSALS

Merger Proposal

As discussed throughout this document, BOCH is asking its shareholders to approve the principal terms of the merger agreement. Holders of BOCH common shares should read this document carefully in its entirety, including the appendices, for more detailed information concerning the merger agreement and the mergers. In particular, holders of BOCH common shares are directed to the merger agreement, a copy of which is attached as Appendix A to this proxy statement/prospectus.

The BOCH board of directors unanimously recommends a vote “FOR” the merger proposal.

Merger-Related Named Executive Officer Compensation Proposal

As required by applicable laws and regulations, BOCH is submitting to a non-binding, advisory shareholder vote the compensation of BOCH’s named executive officers that is based on or otherwise relates to the mergers as described in “The Mergers—Interests of BOCH’s Directors and Executive Officers in the Mergers” beginning on page 39 and “Merger-Related Compensation for BOCH’s Named Executive Officers” beginning on page 42. The proposal gives BOCH’s shareholders the opportunity to express their views on the merger-related compensation of BOCH’s named executive officers. Accordingly, BOCH is requesting that shareholders approve the following resolution, which approval will be on a non-binding, advisory basis:

“RESOLVED, that the compensation that may be paid or become payable to BOCH’s named executive officers in connection with the mergers, and the agreements or understandings pursuant to which such compensation may be paid or become payable, in each case as disclosed pursuant to Item 402(t) of Regulation S-K in ‘The Mergers—Interests of BOCH’s Directors and Executive Officers in the Mergers’ and ‘Merger-Related Compensation for BOCH’s Named Executive Officers,’ are hereby APPROVED.”

The vote on this proposal is a vote separate and apart from the vote to approve the principal terms of the merger agreement. Accordingly, you may vote not to approve this proposal on merger-related named executive officer compensation and vote to approve the principal terms of the merger agreement, or vice versa. You also may abstain from this proposal and vote on the merger proposal, or vice versa. Because the vote is advisory in nature, the outcome will not be binding on BOCH, regardless of whether the merger agreement is approved. Approval of the non-binding, advisory proposal with respect to the compensation that may be paid or become payable to BOCH’s named executive officers in connection with the mergers is not a condition to completion of the mergers, and failure to approve this advisory matter will have no effect on the vote to approve the principal terms of the merger agreement. The merger-related named executive officer compensation that may be paid or become payable in connection with the mergers is based on contractual arrangements with the named executive officers and, accordingly, the outcome of this advisory vote will not affect any contractual obligation to make these payments.

The BOCH board of directors unanimously recommends a vote “FOR” the merger-related named executive officer compensation proposal.

Adjournment Proposal

The BOCH special meeting may be adjourned to another time or place, if necessary or appropriate, to solicit additional proxies if, at the time of the BOCH special meeting, the votes received in favor of the merger proposal are insufficient to approve the proposal in accordance with applicable law and BOCH’s governing documents.

If, at the BOCH special meeting, the number of BOCH common shares present or represented and voting in favor of the merger proposal is insufficient to approve the principal terms of the merger agreement, BOCH

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intends to move to adjourn the BOCH special meeting in order to enable the BOCH board of directors to solicit additional proxies for approval of the merger proposal. In that event, BOCH will ask its shareholders to vote only upon the adjournment proposal at that time, and not on the merger proposal or the merger-related named executive officer compensation proposal.

In this proposal, BOCH is asking its shareholders to authorize the holder of any proxy solicited by the BOCH board of directors to vote in favor of granting discretionary authority to the proxy holders, and each of them individually, to adjourn the BOCH special meeting to another time and place for the purpose of soliciting additional proxies. If the BOCH shareholders approve the adjournment proposal, BOCH may adjourn the BOCH special meeting and use the additional time to solicit more proxies, including the solicitation of proxies from BOCH shareholders who have previously voted.

The BOCH board of directors unanimously recommends a vote “FOR” the adjournment proposal.

Other Matters to Come Before the BOCH Special Meeting

No other matters are intended to be brought before the BOCH special meeting by BOCH, and BOCH does not know of any matters to be brought before the BOCH special meeting by others. If, however, any other matters properly come before the BOCH special meeting, the persons named in the proxy will vote the shares represented thereby in accordance with the judgment of management on any such matter.

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LEGAL OPINIONS

Sullivan & Cromwell LLP and Miller Nash LLP will deliver at the effective time of the parent merger their opinions to Columbia and BOCH, respectively, as to certain United States federal income tax consequences of the parent merger. Please see the section entitled “Material U.S. Federal Income Tax Consequences of the Parent Merger” beginning on page 59. The validity of the Columbia common shares to be issued in the parent merger will be passed upon for Columbia by Kumi Y. Baruffi, Executive Vice President and General Counsel of Columbia.

EXPERTS

The consolidated financial statements, incorporated in this proxy statement/prospectus by reference from Columbia’s Annual Report on Form 10-K for the year ended December 31, 2020, and the effectiveness of Columbia’s internal control over financial reporting have been audited by Deloitte & Touche LLP, an independent registered public accounting firm, as stated in their reports, which are incorporated herein by reference. Such consolidated financial statements have been so incorporated in reliance upon the reports of such firm given upon their authority as experts in accounting and auditing.

BOCH’s consolidated financial statements appearing in its Annual Report on Form 10-K for the year ended December 31, 2020, have been audited by Moss Adams LLP, an independent registered public accounting firm, as stated in their report included therein, which is incorporated herein by reference. Such consolidated financial statements are incorporated herein by reference in reliance upon the report of such firm given upon the authority of such firm as experts in accounting and auditing.

BOCH 2022 ANNUAL MEETING SHAREHOLDER PROPOSALS

BOCH intends to hold an annual meeting of shareholders in 2022 only if the mergers are not completed. In the event that the mergers are not completed in early 2022, or at all, any proposals by shareholders to transact business at BOCH’s 2022 annual meeting of shareholders must be delivered in writing to BOCH at its principal administrative office located at 1901 Churn Creek Road, Redding, California 96002 no later than December 7, 2021 in order to be considered for inclusion in the proxy statement and proxy card. Such proposals must comply with the provisions of BOCH’s bylaws and the SEC’s regulations regarding the inclusion of shareholder proposals in BOCH’s sponsored proxy materials. If a proposal is presented at the 2022 annual meeting of shareholders in compliance with this paragraph, your proxy holder will vote in accordance with the recommendation of the BOCH board of directors or, if no recommendation is given, in accordance with his or her best judgment.

Notice of any business item proposed to be brought from the floor by a shareholder at BOCH’s 2022 annual meeting of shareholders, including the nomination of directors, must be received in writing by the Corporate Secretary of BOCH no earlier than January 18, 2022 and no later than February 17, 2022, must include a brief description of the business desired to be brought before the meeting, and must comply with the provisions of BOCH’s bylaws. If BOCH does not receive timely notice, such proposal will not be considered a business item at the annual meeting of shareholders, and the Chairman of the meeting will refuse to acknowledge any proposal not made in compliance with the foregoing procedures.

If the mergers are completed, BOCH shareholders will become shareholders of Columbia. Any shareholder nominations or proposals which a shareholder wishes to have included in Columbia’s proxy statement and form of proxy relating to its 2022 annual meeting of shareholders must be received by the date, and must otherwise comply with the requirements, described in the Definitive Proxy Statement on Schedule 14A for Columbia’s 2021 Annual Meeting of Shareholders.

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DOCUMENTS INCORPORATED BY REFERENCE

The SEC allows Columbia and BOCH to “incorporate by reference” information into this proxy statement/prospectus, which means that the companies can disclose important information to you by referring you to another document filed separately by them with the SEC. The information incorporated by reference is deemed to be part of this proxy statement/prospectus, except for any information superseded by any information in this proxy statement/prospectus.

This document incorporates by reference the following documents that have previously been filed with the SEC by Columbia:

This document also incorporates by reference the following documents that have previously been filed with the SEC by BOCH:

In addition, Columbia and BOCH are incorporating by reference any documents they may file under Section 13(a), 13(c), 14 or 15(d) of the Exchange Act after the date of this document and prior to the date of the BOCH special meeting; provided, however, that Columbia and BOCH are not incorporating by reference any information furnished (but not filed), except as otherwise specified herein.

Both Columbia and BOCH file annual, quarterly and special reports, proxy statements and other business and financial information with the SEC. You may obtain the information incorporated by reference and any other materials Columbia or BOCH file with the SEC without charge by following the instructions in the section entitled “Where You Can Find More Information” in the forepart of this document.

Neither Columbia nor BOCH has authorized anyone to give any information or make any representation about the mergers or its companies that is different from, or in addition to, that contained in this document or in any of the materials that have been incorporated into this document. Therefore, if anyone does give you information of this sort, you should not rely on it. If you are in a jurisdiction where offers to exchange or sell, or solicitations of offers to exchange or purchase, the securities offered by this document or the solicitation of proxies is unlawful, or if you are a person to whom it is unlawful to direct these types of activities, then the offer presented in this document does not extend to you. The information contained in this document speaks only as of the date of this document unless the information specifically indicates that another date applies.

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APPENDIX A

EXECUTED

AGREEMENT AND PLAN OF MERGER

by and between

COLUMBIA BANKING SYSTEM, INC.

AND

BANK OF COMMERCE HOLDINGS

Dated as of June 23, 2021

TABLE OF CONTENTS

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A-ii