Form S-4/A GOVERNMENT PROPERTIES

SUMMARY

        The following summary highlights some of the information contained in this joint proxy statement/prospectus. This summary may not contain all of the information that may be important to you. For a more complete description of the Merger Agreement, the Merger and the other Transactions, you should carefully read this joint proxy statement/prospectus in its entirety, including the attached Annexes. You should read the information incorporated by reference into this joint proxy statement/prospectus, which includes important business and financial information about GOV and SIR that has been filed with the Securities and Exchange Commission, or the SEC. You may obtain the information incorporated by reference into this joint proxy statement/prospectus without charge by following the instructions in the section entitled "Where You Can Find More Information" beginning on page 172 of this joint proxy statement/prospectus.

Information about the Companies (See page 58)

Government Properties Income Trust

        GOV is a real estate investment trust, or REIT, organized under Maryland law that has qualified to be taxed as a REIT under the IRC. GOV primarily owns properties located throughout the United States that are majority leased to government tenants and office properties located in the metropolitan Washington, D.C. market area that are leased to government and private sector tenants. GOV has no employees. Services which would otherwise be provided to GOV by employees are provided by The RMR Group LLC, or RMR LLC, GOV's manager, and by GOV's managing trustees and officers.

        As of September 30, 2018, GOV wholly owned 105 properties (164 buildings) and had a noncontrolling ownership interest in two properties (three buildings) totaling approximately 0.4 million rentable square feet through two unconsolidated joint ventures in which GOV owned 50% and 51% interests. As of September 30, 2018, GOV's consolidated properties were located in 30 states and the District of Columbia and contained approximately 17.0 million rentable square feet, of which 42.0% was leased to the United States Government, 14.7% was leased to 13 state governments, 1.5% was leased to three other government tenants, 5.8% was leased to government contractor tenants, 29.3% was leased to various other non-governmental organizations and 6.7% was available for lease. As of September 30, 2018, GOV's consolidated properties had a weighted average (based on annualized rental income) remaining lease term of approximately 4.4 years. In the aggregate, government tenants were responsible for 62.5% of GOV's annualized rental income as of September 30, 2018. The above information does not give effect to the Other Asset Dispositions. GOV defines the term annualized rental income as the annualized contractual base rents from GOV's tenants pursuant to its lease agreements as of the measurement date, plus straight line rent adjustments and estimated recurring expense reimbursements to be paid to GOV, and excluding lease value amortization.

        As of September 30, 2018, GOV owned 24,918,421 SIR Common Shares, or approximately 27.8% of the SIR Common Shares outstanding as of such date. On October 9, 2018, as required by the Merger Agreement, GOV sold all 24,918,421 SIR Common Shares owned by GOV in an underwritten public offering at a price of $18.25 per share, raising net proceeds of approximately $434.7 million after deducting underwriting discounts and estimated offering expenses.

        GOV's principal executive offices are located at Two Newton Place, 255 Washington Street, Suite 300, Newton, Massachusetts 02458, and its telephone number is (617) 219-1440. GOV maintains a website at www.govreit.com where additional information regarding GOV can be found. The contents of that website are not incorporated by reference into, or otherwise a part of, this joint proxy statement/prospectus.

        Additional information about GOV and its subsidiaries is included in documents incorporated by reference into this joint proxy statement/prospectus. See "Where You Can Find More Information" beginning on page 172 of this joint proxy statement/prospectus.

Select Income REIT

        SIR is a REIT organized under Maryland law that has qualified to be taxed as a REIT under the IRC. SIR owns properties that are primarily net leased to single tenants. SIR has no employees. Services which would otherwise be provided to SIR by employees are provided by RMR LLC, SIR's manager, and by SIR's managing trustees and officers.

        As of September 30, 2018, SIR's consolidated portfolio consisted of 368 buildings, leasable land parcels and easements with approximately 45.8 million rentable square feet that were approximately 95.8% leased (based on rentable square feet). As of September 30, 2018, SIR's consolidated properties were leased to 328 different tenants, with a weighted average (based on annualized rental income) remaining lease term of approximately 8.6 years. The above information is for SIR, on a consolidated basis, including ILPT. SIR defines the term annualized rental income as the annualized contractual rents, as of September 30, 2018, including straight line rent adjustments and excluding lease value amortization, adjusted for tenant concessions including free rent and amounts reimbursed to tenants, plus estimated recurring expense reimbursements from tenants.

        As of September 30, 2018, ILPT owned 269 buildings, leasable land parcels and easements with approximately 29.2 million rentable square feet, including 226 buildings, leasable land parcels and easements with approximately 16.8 million rentable square feet located on the island of Oahu, HI, and 43 buildings with approximately 12.4 million rentable square feet located in 25 other states. As of September 30, 2018, ILPT's properties were approximately 99.3% leased (based on rentable square feet) to 245 different tenants with a weighted average (based on annualized rental income) remaining lease term of approximately 10.9 years. ILPT defines the term annualized rental income as the annualized contractual rents, as of September 30, 2018, including straight line rent adjustments and excluding lease value amortization, adjusted for tenant concessions including free rent and amounts reimbursed to tenants, plus estimated recurring expense reimbursements from tenants. These amounts are included in SIR's consolidated portfolio amounts described above.

        As of September 30, 2018, SIR owned 45,000,000 ILPT Common Shares, or approximately 69.2% of the outstanding ILPT Common Shares as of such date. As a condition to the closing of the Merger, following the Secondary Sale, the receipt of the GOV and SIR shareholder approvals described in this joint proxy statement/prospectus and subject to the satisfaction or waiver of certain other conditions, SIR will declare and, at least one business day prior to the closing date of the Merger, pay the ILPT Distribution.

        SIR's principal executive offices are located at Two Newton Place, 255 Washington Street, Suite 300, Newton, Massachusetts 02458, and its telephone number is (617) 796-8303. SIR maintains a website at www.sirreit.com where additional information regarding SIR can be found. The contents of that website are not incorporated by reference into, or otherwise a part of, this joint proxy statement/prospectus.

        Additional information about SIR and its subsidiaries is included in documents incorporated by reference into this joint proxy statement/prospectus. See "Where You Can Find More Information" beginning on page 172 of this joint proxy statement/prospectus.

The Combined Company

        Based on GOV and SIR (excluding ILPT) properties owned as of September 30, 2018, and after giving effect to the Other Asset Dispositions and the market price of GOV Common Shares as of November 7, 2018, the combined company is expected to own 160 properties (213 buildings) located in 38 states and the District of Columbia containing 30.2 million rentable square feet, and have undepreciated gross assets of $5.7 billion and total assets of $5.3 billion. As of September 30, 2018, as adjusted as described in the preceding sentence, these properties were 92% leased (based on rentable square feet) to 411 different tenants, and the average remaining lease term of these properties,

weighted by annualized rental income, was 5.8 years, with leases expiring between 2018 and 2021 contributing approximately 29% of the combined company's annualized rental income.

The Merger and the Merger Agreement (See pages 82 and 143)

The Merger Agreement

        GOV, GOV Merger Sub and SIR have entered into the Merger Agreement, a copy of which is attached as Annex A to this joint proxy statement/prospectus. Each of the GOV board of trustees and the SIR board of trustees has unanimously approved the Merger Agreement, the Merger and the other transactions contemplated by the Merger Agreement to which it is a party. GOV and SIR encourage you to carefully read the Merger Agreement in its entirety because it is the primary legal document governing the Merger.

Form of the Merger

        Pursuant to the Merger Agreement, SIR will merge with and into GOV Merger Sub, with GOV Merger Sub as the surviving entity in the Merger. Following the Merger, GOV expects GOV Merger Sub to merge with and into GOV, with GOV as the surviving entity, and GOV will change its name to "Office Properties Income Trust" and change its ticker symbol on Nasdaq to "OPI".

        Upon consummation of the Merger, based upon the number of GOV Common Shares and SIR Common Shares outstanding as of the date of this joint proxy statement/prospectus, we estimate that the GOV shareholders immediately prior to the Merger (in their capacities as such) will own approximately 51.6% of the GOV Common Shares outstanding as a result of the Merger and the SIR shareholders immediately prior to the Merger (in their capacities as such) will own approximately 48.4% of the GOV Common Shares outstanding as a result of the Merger, in each case without taking into account whether any of those GOV shareholders were also SIR shareholders at that time.

Consideration to SIR Shareholders in the Merger

        Pursuant to the Merger Agreement, at the Effective Time, each SIR Common Share (other than any shares owned by GOV or SIR or their respective wholly owned subsidiaries and in each case not held on behalf of third parties) outstanding immediately prior to the Effective Time will be converted into the right to receive 1.04 newly issued GOV Common Shares, with cash paid in lieu of fractional shares. The exchange ratio in the Merger is fixed and will not be adjusted for changes in the market price of GOV Common Shares or SIR Common Shares. Because of this, the implied value of the Merger Consideration to SIR shareholders will fluctuate between now and the date of the consummation of the Merger. Based on the $16.89 closing price of GOV Common Shares on Nasdaq on September 14, 2018, the last trading day before the public announcement of the Merger, the exchange ratio represented approximately $17.57 in GOV Common Shares for each SIR Common Share. Based on the $9.03 closing price of GOV Common Shares on Nasdaq on November 14, 2018, the latest practicable date before the date of this joint proxy statement/prospectus, the exchange ratio represented approximately $9.39 in GOV Common Shares for each SIR Common Share. For more information, see "—Comparative GOV and SIR Market Price and Distribution Information" beginning on page 41 of this joint proxy statement/prospectus.

        The following table presents trading information for GOV Common Shares and SIR Common Shares on September 14, 2018, the last trading day before the public announcement of the Merger, and November 14, 2018, the latest practicable date before the date of this joint proxy statement/prospectus.

Equivalent per share value of the Merger Consideration for SIR shareholders, giving effect to the exchange ratio, is also provided for each of these dates.

        The value of the Merger Consideration will fluctuate with changes in the market price of GOV Common Shares. The sales prices above for SIR reflect the consolidated operations of SIR, including ILPT; however, the ILPT Distribution is a condition to the closing of the Merger. If the ILPT Distribution and the Merger are completed, the Merger Consideration that existing SIR shareholders will be entitled to receive will be in addition to any ILPT Common Shares that they receive in the ILPT Distribution.

Treatment of Unvested SIR Share Awards in the Merger

        In connection with the entry into the Merger Agreement, SIR obtained from each of its executive officers and all executive officers of RMR LLC a waiver of each such officer's right to accelerated vesting upon consummation of the Merger of any unvested or partially vested SIR Common Shares subject to vesting restrictions, or SIR Share Awards, such officer held. At the Effective Time, each unvested or partially vested SIR Share Award will be converted into an award under GOV's 2009 Incentive Share Award Plan, or a Rollover Equity Award, with respect to a number of GOV Common Shares (rounded down to the nearest whole share) equal to the product of (i) the exchange ratio, multiplied by (ii) the number of SIR Common Shares subject to such unvested or partially vested SIR Share Award at the Effective Time, with cash paid in lieu of fractional shares. Such Rollover Equity Award will continue to be subject to the same vesting and other terms and conditions as were in effect immediately prior to the Effective Time, except as specifically set forth in the waiver or as otherwise amended. All amounts payable in respect of such SIR Share Awards will be subject to appropriate tax withholding.

The ILPT Distribution (See page 163)

        Pursuant to the Merger Agreement, SIR has agreed that, subject to the satisfaction or waiver of certain conditions, it will declare and, at least one business day prior to the closing date of the Merger, pay the ILPT Distribution. If the Merger is completed, the Merger Consideration that existing SIR shareholders are entitled to receive will be in addition to any ILPT Common Shares they receive in the ILPT Distribution. Based on the number of SIR Common Shares outstanding as of the date of this joint proxy statement/prospectus, SIR shareholders are expected to receive approximately 0.503 ILPT Common Shares for each SIR Common Share.

        After the ILPT Distribution is declared, SIR will provide its shareholders with additional information. The income tax characterization of the ILPT Distribution will not be ultimately determined until after the close of SIR's taxable year in which the ILPT Distribution has occurred. However, assuming that the ILPT Distribution occurs in 2018 and that the price of the ILPT Common Shares when distributed is approximately the same as on November 7, 2018, then it is expected that approximately half of SIR's cash and other distributions for all of 2018 (including the ILPT Distribution) will be a nontaxable return of capital, and the balance will largely be a capital gain dividend from the sale of securities that are not United States real property interests, with approximately 5% of each distribution comprising an ordinary REIT dividend. Nontaxable returns of capital are generally nontaxable to both U.S. and non-U.S. shareholders, while capital gain dividends from the sale of securities that are not United States real property interests are generally nontaxable to non-U.S. shareholders and taxable at preferential rates to certain U.S. shareholders; nevertheless, because withholding rates and policies are complex and very dependent upon the discretion of

applicable withholding agents, withholding may apply to SIR's cash and other distributions (including the ILPT Distribution) in excess of the substantive tax liability applicable to such distributions, and the applicable shareholder may have to claim a refund from the IRS to receive the overwithheld amounts.

Registration Agreement (See page 163)

        Pursuant to the Merger Agreement, GOV and SIR agreed that, prior to the Effective Time, GOV will complete the Secondary Sale. Concurrently with the execution and delivery of the Merger Agreement, GOV and SIR entered into a registration agreement, or the Registration Agreement, pursuant to which GOV received the right to demand registration, subject to certain limitations, of the resale of the 24,918,421 SIR Common Shares owned by GOV. Under the Registration Agreement, GOV agreed to pay all of the costs and expenses incurred in connection with the Secondary Sale, including costs and expenses of SIR and its affiliates. On October 9, 2018, pursuant to the terms of the Registration Agreement, GOV sold all of the SIR Common Shares it beneficially owned.

Management Agreements (See page 163)

        GOV and SIR each have a business management agreement and a property management agreement with RMR LLC. Contemporaneously with the execution and delivery of the Merger Agreement, RMR LLC and SIR entered into a letter agreement, or the RMR LLC Letter Agreement, pursuant to which, on the terms and subject to the conditions set forth therein, they agreed that, effective upon consummation of the Merger, SIR shall have terminated its business and property management agreements with RMR LLC for convenience, and RMR LLC shall have waived its right to receive payment of the termination fee due on account thereof. If the Merger is consummated, GOV's business management and property management agreements will remain in effect and apply to the combined company. See "The Merger Agreement—Covenants and Agreements—RMR LLC Management Agreements" beginning on page 156 of this joint proxy statement/prospectus. For a description of GOV's management agreements, see "Management Agreements" beginning on page 163 of this joint proxy statement/prospectus.

Recommendations of the GOV Board of Trustees (See page 72)

        After careful consideration, the GOV board of trustees, on September 14, 2018, unanimously approved the Merger Agreement, the Merger and the other transactions contemplated by the Merger Agreement to which GOV or GOV Merger Sub is a party and declared the Merger Agreement and such other transactions to be fair and reasonable and advisable to, and in the best interests of, GOV.

        The GOV board of trustees unanimously recommends that GOV shareholders vote "FOR" the GOV Share Issuance Proposal and "FOR" the GOV Adjournment Proposal.

        For the factors considered by the GOV board of trustees in reaching its decision to approve the Merger Agreement and make the foregoing recommendations, see "The Merger—Recommendation of the GOV Board of Trustees and its Reasons for the Merger and the Other Transactions" beginning on page 93 of this joint proxy statement/prospectus.

Recommendations of the SIR Board of Trustees (See page 77)

        After careful consideration, the SIR board of trustees, on September 14, 2018, unanimously approved the Merger Agreement, the Merger and the other transactions contemplated by the Merger Agreement to which SIR is a party and declared the Merger Agreement and such other transactions to be fair and reasonable and advisable to, and in the best interests of, SIR.

        The SIR board of trustees unanimously recommends that SIR Shareholders vote "FOR" the SIR Merger Proposal and "FOR" the SIR Adjournment Proposal.

        For the factors considered by the SIR board of trustees in reaching its decision to approve the Merger Agreement and make the foregoing recommendations, see "The Merger—Recommendation of the SIR Board of Trustees and its Reasons for the Merger and the Other Transactions" beginning on page 100 of this joint proxy statement/prospectus.

Risks Relating to the Merger and the Other Transactions (See page 43)

        You should carefully consider all the risk factors together with all of the other information in this joint proxy statement/prospectus before deciding how to vote. The risks relating to the Merger and the other Transactions are described under the caption "Risk Factors—Risks Relating to the Merger and the Other Transactions" beginning on page 43 of this joint proxy statement/prospectus. The principal risks relating to the Merger and the Other Transactions include the following:

•

The exchange ratio is fixed and will not be adjusted for any changes in the market price of either GOV Common Shares or SIR Common Shares.



•

The Merger and the other Transactions are subject to the satisfaction or waiver of a number of conditions, including certain conditions that may not be satisfied, or completed on a timely basis, if at all. Failure to complete these transactions could have material and adverse effects on SIR or GOV.



•

GOV or SIR may waive one or more of the conditions to the Merger or the other Transactions without re-soliciting shareholder approval.



•

The Merger Agreement contains provisions that could discourage a potential competing acquirer of either GOV or SIR or could result in any competing proposal being at a lower price than it might otherwise be.



•

GOV's, SIR's and ILPT's business management agreements with RMR LLC contain provisions that could discourage a potential competing acquirer of either GOV or SIR or could result in any competing proposal being at a significantly lower price than it might otherwise be.



•

GOV's ownership of SIR Common Shares as of the SIR Record Date could discourage a potential competing acquirer of SIR.



•

Completion of the ILPT Distribution is not conditioned upon consummation of the Merger. As a result, there is a risk that the ILPT Distribution will be completed and the Merger not consummated, leaving SIR a significantly smaller company that will no longer have an equity interest in or any control over ILPT, benefit from ILPT's income or receive distributions from ILPT.



•

The pendency of the Merger and the other Transactions could adversely affect the business and operations of GOV and SIR.



•

GOV and SIR shareholders will be diluted by the consummation of the Merger.



•

Certain of GOV's and SIR's trustees and executive officers and their manager, RMR LLC, have interests in the Merger and the other Transactions that are different from, or in addition to, the interests of GOV and SIR shareholders generally, which may create potential conflicts of interest or the appearance thereof.



•

The interests of certain of GOV's and SIR's trustees and executive officers and RMR LLC in the Merger and the other Transactions may lead to increased dissident shareholder activity, including litigation, related to the Merger Agreement and the Transactions, which could result in significant costs for GOV and SIR and materially delay or prevent the completion of the Merger and the other Transactions.



•

The unaudited pro forma condensed consolidated financial information in this joint proxy statement/prospectus is presented for illustrative purposes only and may not be reflective of the

operating results and financial condition of the combined company following consummation of the Merger and the other Transactions.

•

SIR shareholders who receive ILPT Common Shares in the ILPT Distribution may not realize the anticipated benefits of such distribution.

Shareholders Entitled to Vote; Votes Required (See page 73)

Government Properties Income Trust

        GOV shareholders who owned GOV Common Shares at the close of business on the GOV Record Date are entitled to notice of, and to vote at, the GOV special meeting. On the GOV Record Date, there were 99,205,199 GOV Common Shares issued and outstanding and entitled to vote at the GOV special meeting, held by approximately 212 holders of record. Each GOV Common Share is entitled to one vote on each of the GOV Share Issuance Proposal and the GOV Adjournment Proposal.

        At the GOV special meeting, the presence in person or by proxy of shareholders entitled to cast a majority of all the votes entitled to be cast at such meeting shall constitute a quorum. Abstentions and broker non-votes, if any, will be included in determining whether a quorum is present at the GOV special meeting.

        The GOV Share Issuance Proposal and the GOV Adjournment Proposal require the affirmative vote of at least a majority of all the votes cast by holders of outstanding GOV Common Shares entitled to vote at the GOV special meeting on such proposals. Pursuant to GOV's Amended and Restated Bylaws, or the GOV Bylaws, the chairperson of the GOV special meeting may adjourn the GOV special meeting to a later date or dates, for any reason deemed necessary by the chairperson, without GOV shareholder approval.

        See page 73 of this joint proxy statement/prospectus for a description of the effect of abstentions and broker non-votes with respect to the above proposals.

        Your vote is very important. Please complete, sign, date and return your proxy card as promptly as possible. If you properly submit your proxy card but do not indicate how your GOV Common Shares should be voted on a matter, the GOV Common Shares represented by your properly completed proxy will be voted as the GOV board of trustees unanimously recommends and therefore "FOR" the GOV Share Issuance Proposal and "FOR" the GOV Adjournment Proposal. If you do not provide voting instructions to your broker, bank or other nominee, your GOV Common Shares will NOT be voted on such proposals at the meeting and will be considered broker non-votes with respect to such proposals.

Select Income REIT

        SIR shareholders who owned SIR Common Shares at the close of business on the SIR Record Date are entitled to notice of, and to vote at, the SIR special meeting. On the SIR Record Date, there were 89,550,528 SIR Common Shares issued and outstanding and entitled to vote at the SIR special meeting, held by approximately 2,386 holders of record, including GOV, which owned 24,918,421 SIR Common Shares, or approximately 27.8% of the SIR Common Shares outstanding on the SIR Record Date. Each SIR Common Share is entitled to one vote on each of the SIR Merger Proposal and the SIR Adjournment Proposal.

        At the SIR special meeting, the presence in person or by proxy of shareholders entitled to cast a majority of all the votes entitled to be cast at such meeting shall constitute a quorum. Abstentions and broker non-votes, if any, will be included in determining whether a quorum is present at the SIR special meeting.

        The SIR Merger Proposal requires the affirmative vote of at least a majority of all the votes entitled to be cast by holders of outstanding SIR Common Shares at the SIR special meeting on such proposal. The SIR Adjournment Proposal requires the affirmative vote of at least a majority of all the

votes cast by holders of outstanding SIR Common Shares entitled to vote at the SIR special meeting on such proposal. Pursuant to SIR's Amended and Restated Bylaws, or the SIR Bylaws, the chairperson of the SIR special meeting may adjourn the SIR special meeting to a later date or dates, for any reason deemed necessary by the chairperson, without SIR shareholder approval.

        See page 78 of this joint proxy statement/prospectus for a description of the effect of abstentions and broker non-votes with respect to the above proposals.

        Your vote is very important. Please complete, sign, date and return your proxy card as promptly as possible. If you properly submit your proxy card but do not indicate how your SIR Common Shares should be voted on a matter, the SIR Common Shares represented by your properly completed proxy will be voted as the SIR board of trustees unanimously recommends and therefore "FOR" the SIR Merger Proposal and "FOR" the SIR Adjournment Proposal. If you do not provide voting instructions to your broker, bank or other nominee, your SIR Common Shares will NOT be voted on such proposals at the meeting and will be considered broker non-votes with respect to such proposals.

Opinion of Financial Advisor to the GOV Special Committee (See page 106)

        In connection with the Merger, Citigroup Global Markets Inc., or Citi, financial advisor to a special committee of the GOV board of trustees, or the GOV special committee, delivered a written opinion, dated September 14, 2018, to the GOV special committee as to the fairness, from a financial point of view and as of the date of the opinion, to GOV of the exchange ratio provided for pursuant to the Merger Agreement.

        The full text of Citi's written opinion, dated September 14, 2018, which describes the assumptions made, procedures followed, matters considered and limitations and qualifications on the review undertaken, is attached as Annex B to this joint proxy statement/prospectus and is incorporated into this joint proxy statement/prospectus by reference. The description of Citi's opinion set forth below is qualified in its entirety by reference to the full text of Citi's opinion. Citi's opinion was provided for the information of the GOV special committee (in its capacity as such) in connection with its evaluation of the exchange ratio from a financial point of view to GOV and did not address any other terms, aspects or implications of the Merger or the other Transactions. Citi expressed no view as to, and its opinion did not address, the underlying business decision of GOV to effect or enter into the Merger or any of the other Transactions, the relative merits of the Merger or any of the other Transactions as compared to any alternative business strategies that might exist for GOV or the effect of any other transaction which GOV might engage in or consider. Citi's opinion is not intended to be and does not constitute a recommendation to any securityholder as to how such securityholder should vote or act on any matters relating to the proposed Merger, any of the other Transactions or otherwise.

Opinion of Financial Advisor to the SIR Special Committee (See page 116)

        A special committee of the SIR board of trustees, or the SIR special committee, retained UBS Securities LLC, or UBS, as financial advisor to the SIR special committee in connection with the Merger. In connection with this engagement, the SIR special committee requested that UBS evaluate the fairness, from a financial point of view, to the holders of SIR Common Shares (other than holders of excluded shares (as defined in the Merger Agreement)), of the exchange ratio. On September 14, 2018, UBS rendered to the SIR special committee and the SIR board of trustees its oral opinion, which was subsequently confirmed by delivery of a written opinion, to the effect that, as of such date and based upon and subject to the factors, procedures, assumptions, qualifications and limitations set forth in UBS' written opinion, the exchange ratio was fair, from a financial point of view, to the holders of SIR Common Shares (other than holders of excluded shares (as defined in the Merger Agreement)).

        The full text of UBS' opinion describes the assumptions made, procedures followed, matters considered and qualifications and limitations on the review undertaken by UBS. UBS' opinion is

attached as Annex C to this joint proxy statement/prospectus and is incorporated herein by reference. Holders of SIR Common Shares are encouraged to read UBS' opinion carefully in its entirety. UBS' opinion was provided for the benefit of the SIR special committee (in its capacity as such) and the SIR board of trustees (in its capacity as such) in connection with, and for the purpose of, their evaluation of the exchange ratio, and does not address any other aspect of the Merger. UBS' opinion does not address the relative merits of the Merger as compared to other business strategies or transactions that might be available to SIR or SIR's underlying business decision to effect the Merger. UBS' opinion does not constitute a recommendation to any shareholder as to how such shareholder should vote or act with respect to the Merger.

Interests of GOV and SIR Trustees, Executive Officers and Manager in the Merger and the Other Transactions (See page 130)

        Certain of GOV's and SIR's trustees and executive officers and their manager, RMR LLC, have interests in the Merger and the other Transactions that are different from, or in addition to, the interests of GOV and SIR shareholders generally, which may create potential conflicts of interest or the appearance thereof. Each of the GOV and SIR board of trustees and GOV and SIR special committees were aware of these interests, among other matters, in approving the Merger Agreement, the Merger and the other transactions contemplated by the Merger Agreement to which GOV or GOV Merger Sub or SIR, respectively, is a party, and in recommending that GOV shareholders vote for the GOV Share Issuance Proposal and that SIR shareholders vote for the SIR Merger Proposal, respectively. These interests include those discussed below.

        RMR LLC is the business and property manager of GOV, SIR and ILPT, and the personnel and various services that each of GOV, SIR and ILPT requires to operate its respective business are provided to it by RMR LLC pursuant to its respective management agreements with RMR LLC, which have successive 20 year terms. RMR LLC is a majority owned subsidiary of The RMR Group Inc., or RMR Inc., and RMR Inc. is the managing member of RMR LLC. Adam D. Portnoy, a managing trustee of GOV, SIR and ILPT, is the sole trustee, an officer and the controlling shareholder of ABP Trust, which is the controlling shareholder of RMR Inc.; a managing director and the president and chief executive officer of RMR Inc.; and an officer of RMR LLC. GOV's, SIR's and ILPT's other managing trustee and each of their executive officers also serve as executive officers, and are employees, of RMR LLC. Jeffrey P. Somers, an independent trustee of GOV, also serves as an independent trustee of SIR. The completion of the Merger and the other Transactions may increase the aggregate fees payable to RMR LLC by GOV and ILPT under their management agreements with RMR LLC. The termination of GOV's, SIR's or ILPT's business and property management agreements with RMR LLC may require GOV, SIR or ILPT, as applicable, to pay a substantial termination fee to RMR LLC, including in the case of a termination by GOV, SIR or ILPT, as applicable, for convenience or for a performance reason or, in the case of a termination by RMR LLC, for good reason. However, RMR LLC and SIR entered into the RMR LLC Letter Agreement, pursuant to which RMR LLC has agreed to waive its right to receive payment of the termination fee otherwise due under its business and property management agreements with SIR upon the termination of those agreements when the Merger is consummated. This waiver by RMR LLC applies only in respect of the Merger and does not apply in respect of any competing proposal or superior proposal (each as defined in the Merger Agreement) or any other transaction or arrangement.

        GOV's current trustees and executive officers and their affiliates collectively beneficially owned 1,356,277 GOV Common Shares, or approximately 1.37% of the GOV Common Shares outstanding as of the GOV Record Date and have indicated that they expect to vote "FOR" the GOV Share Issuance Proposal and "FOR" the GOV Adjournment Proposal, if necessary or appropriate. For more information, see "The GOV Special Meeting—Required Vote" on page 73 of this joint proxy statement/prospectus. SIR's current trustees and executive officers and their affiliates collectively beneficially owned 1,723,410 SIR Common Shares, or approximately 1.92% of the SIR Common Shares

outstanding as of the SIR Record Date and have indicated that they expect to vote "FOR" the SIR Merger Proposal and "FOR" the SIR Adjournment Proposal, if necessary or appropriate. For more information, see "The SIR Special Meeting—Required Vote" on page 78 of this joint proxy statement/prospectus.

        None of GOV's or SIR's trustees or executive officers has any arrangement or understanding with either GOV or SIR concerning any type of compensation based on the Merger or the other Transactions, but each of GOV's and SIR's trustees and executive officers are entitled to certain rights to indemnification and SIR's trustees and executive officers are entitled to continued directors' and officers' liability insurance coverage following the completion of the Merger.

        As discussed above, the transactions contemplated by the Merger Agreement and the terms thereof were evaluated, negotiated and recommended to each of the GOV board of trustees and the SIR board of trustees by the GOV special committee and the SIR special committee, respectively, each comprised solely of GOV's and SIR's disinterested, independent trustees, respectively, and were separately approved and adopted by GOV's and SIR's independent trustees and by the GOV board of trustees and the SIR board of trustees.

Listing of GOV Common Shares to be Issued in the Merger (See page 135)

        Approval of Nasdaq of the listing on Nasdaq of the GOV Common Shares to be issued in the Merger, subject to official notice of issuance, is a condition precedent to GOV's and SIR's obligations to consummate the Merger. GOV has agreed to take all necessary steps to cause such GOV Common Shares to be listed for trading on Nasdaq, subject to official notice of issuance. If the Merger is consummated, SIR Common Shares will cease to exist and will be de-listed from Nasdaq and de-registered under the Exchange Act.

No Shareholder Appraisal Rights in the Merger (See page 167)

        Under the provisions of Maryland law applicable to Maryland REITs, or the Maryland REIT Law, and pursuant to the declaration of trust of each of GOV and SIR, neither GOV shareholders nor SIR shareholders are entitled to exercise appraisal rights in connection with the Merger or any of the other Transactions.

Litigation Related to the Merger and the Other Transactions (See page 135)

        Certain litigation is pending in connection with the Merger. For more information, see "The Merger—Litigation Related to the Merger and the Other Transactions," on page 135.

Conditions to Completion of the Merger (See page 158)

        As more fully described in this joint proxy statement/prospectus and in the Merger Agreement, the consummation of the Merger depends on a number of conditions being satisfied or, where legally permissible, waived. These conditions include:

•

the approval of the SIR Merger Proposal by SIR shareholders and approval of the GOV Share Issuance Proposal by GOV shareholders;



•

the absence of any law or order by any governmental authority prohibiting, making illegal, enjoining or otherwise restricting, preventing or prohibiting the consummation of the Merger, the GOV Share Issuance or the other Transactions;



•

the SEC having declared effective the registration statement, of which this joint proxy statement/prospectus forms a part, and the registration statement not being the subject of any stop order or proceedings seeking a stop order;



•

the approval of Nasdaq of the listing on Nasdaq of the GOV Common Shares to be issued in the Merger, subject to official notice of issuance;

•

GOV having completed the Secondary Sale, which condition was satisfied on October 9, 2018;



•

SIR having declared and, at least one business day prior to the closing date of the Merger, paid the ILPT Distribution;



•

the accuracy of all representations and warranties made by GOV, GOV Merger Sub and SIR in the Merger Agreement, the performance by the parties of their obligations under the Merger Agreement (subject in most cases to materiality or material adverse effect qualifications), there having not occurred a material adverse effect with respect to GOV or SIR, as applicable, since the date of the Merger Agreement, and receipt of an officer's certificate from each of GOV and SIR attesting to each of the foregoing;



•

receipt by each of GOV and SIR of an opinion of its respective tax counsel to the effect that (i) the Merger will qualify as a reorganization within the meaning of Section 368(a) of the IRC and (ii) GOV and SIR will each be a party to that reorganization within the meaning of Section 368(b) of the IRC; and



•

the receipt by each of GOV and SIR of an opinion of counsel dated as of the closing date of the Merger regarding such party's qualification for taxation as a REIT.

        Neither GOV nor SIR can be sure when, or if, the conditions to the Merger will be satisfied or waived, or that the Merger will be completed.

Regulatory Approvals (See page 134)

        Neither GOV nor SIR is aware of any regulatory approvals that are expected to prevent the consummation of the Merger.

No Solicitation; Change in Recommendation (See page 151)

        Pursuant to the Merger Agreement, GOV and SIR have each agreed not to, and to cause their respective subsidiaries (excluding, in the case of SIR, ILPT and its subsidiaries) not to, directly or indirectly, (a) solicit, initiate or knowingly facilitate or knowingly encourage, any inquiry or the making of any proposal which constitutes, or may reasonably be expected to lead to, a competing acquisition proposal, (b) engage in, continue or otherwise participate in any discussions or negotiations regarding, or furnish to any other person or entity information in connection with or for the purpose of encouraging or facilitating, a competing acquisition proposal or (c) enter into any letter of intent, memorandum of understanding, merger agreement, acquisition agreement, agreement in principle or other agreement with respect to a competing acquisition proposal. GOV and SIR have also agreed to, and to cause their respective subsidiaries (excluding, in the case of SIR, ILPT and its subsidiaries) to, immediately cease any solicitation, encouragement, discussions or negotiations that may be ongoing with any third parties with respect to a competing acquisition proposal.

        However, prior to the approval of the GOV Share Issuance Proposal by GOV shareholders, in the case of GOV, and prior to the approval of the SIR Merger Proposal by SIR shareholders, in the case of SIR, GOV and SIR may, under certain specified circumstances, engage in discussions or negotiations with and provide non-public information regarding itself to a third party making a confidential unsolicited competing acquisition proposal. Under the Merger Agreement, if GOV or SIR receives an unsolicited competing acquisition proposal, it is required to notify the other party promptly.

        Prior to the approval of the GOV Share Issuance Proposal by GOV shareholders, in the case of GOV, and prior to the approval of the SIR Merger Proposal by SIR shareholders, in the case of SIR, the GOV board of trustees may withdraw its recommendation to approve the GOV Share Issuance Proposal (and the GOV special committee may withdraw its recommendation to the GOV board of trustees with respect to the Merger and the other Transactions) and the SIR board of trustees may withdraw its recommendation to approve the SIR Merger Proposal (and the SIR special committee may withdraw its recommendation to the SIR board of trustees with respect to the Merger and the

other Transactions) (a) in order to enter into an agreement for a transaction that constitutes a superior proposal and only then if the applicable board of trustees determines in good faith, after consultation with outside legal counsel and financial advisors, that such transaction constitutes a superior proposal or (b) if an intervening event not known or reasonably foreseeable on the date of the Merger Agreement occurs with respect to such party and only then if the applicable board of trustees determines in good faith, after consultation with outside legal counsel, that failure to take such action would be inconsistent with the trustees' duties under applicable Maryland law.

Trustees and Management of the Combined Company (See page 133)

        Pursuant to the Merger Agreement, the trustees and officers of GOV Merger Sub immediately after the Effective Time, will continue as the trustees and officers of GOV Merger Sub. GOV expects that, following the consummation of the Merger, Messrs. Adam D. Portnoy and David M. Blackman will serve as the managing trustees of the board of trustees of OPI, and Barbara D. Gilmore, John L. Harrington, Elena Poptodorova and Jeffrey P. Somers will continue to serve as independent trustees of the board of trustees of OPI. The GOV board of trustees plans, effective upon consummation of the Merger, to increase the size of the board of trustees of OPI from six members to eight members, and to invite Donna D. Fraiche and William A. Lamkin, independent trustees of SIR, to join the board of trustees of OPI as independent trustees.

        On October 25, 2018, GOV announced that Mark L. Kleifges was retiring as a managing trustee and the chief financial officer and treasurer of GOV, effective December 31, 2018, and that, effective January 1, 2019, David M. Blackman, the current president and chief executive officer of GOV, will replace Mr. Kleifges as a managing trustee of GOV, and Jeffrey C. Leer will replace Mr. Kleifges as the chief financial officer and treasurer of GOV. Also on October 25, 2018, SIR announced that John C. Popeo was retiring as the chief financial officer and treasurer of SIR, effective November 30, 2018, and that, effective December 1, 2018, Jeffrey C. Leer will replace Mr. Popeo as the chief financial officer and treasurer of SIR. Mr. Blackman and Mr. Leer will serve as the president and chief executive officer and chief financial officer and treasurer, respectively, of OPI.

Expected Timing of the Merger

        GOV and SIR expect to consummate the Merger by December 31, 2018. However, the Merger is subject to various conditions, and it is possible that factors beyond GOV's and SIR's control could result in the Merger being completed at a later time, or not at all. For more information, see "Risk Factors—Risks Relating to the Merger and the Other Transactions" beginning on page 43 of this joint proxy statement/prospectus.

Termination of the Merger Agreement (See page 160)

        The Merger Agreement may be terminated at any time before the Effective Time by the mutual written agreement of GOV and SIR.

        In addition, either GOV or SIR (so long as it is not at fault) may terminate the Merger Agreement:

•

if the Merger is not completed by June 30, 2019;



•

if a governmental authority of competent jurisdiction has issued a final and non-appealable order, decree or ruling permanently restraining, enjoining or otherwise prohibiting the Merger or any of the other Transactions;



•

if GOV shareholders fail to approve the GOV Share Issuance Proposal; or



•

if SIR shareholders fail to approve the SIR Merger Proposal.

        GOV may terminate the Merger Agreement:

•

if SIR has breached, violated or failed to perform any of its representations, warranties, covenants or agreements set forth in the Merger Agreement which breach, violation or failure to perform would result in a failure of GOV's conditions to the consummation of the Merger and SIR does not cure such breach within a specified period; provided that GOV is not in breach of any of its representations, warranties, covenants or agreements set forth in the Merger Agreement which breach would result in a failure of SIR's conditions to the consummation of the Merger; or



•

if, prior to the receipt of the requisite approval of SIR shareholders, the SIR board of trustees (or an authorized committee thereof) (a) has made an adverse recommendation change, (b) fails to publicly reaffirm the SIR board of trustees' recommendation for the SIR Merger Proposal within ten days of being requested to do so by GOV following the public announcement by any person of a competing acquisition proposal, (c) fails to include its recommendation for the SIR Merger Proposal in this joint proxy statement/prospectus or (d) publicly announces its intention to do any of the foregoing; or



•

if, prior to the receipt of the requisite approval of GOV shareholders, the GOV board of trustees (or an authorized committee thereof) determines to enter into an acquisition agreement with respect to a superior proposal pursuant to the Merger Agreement.

        SIR may terminate the Merger Agreement:

•

if GOV has breached, violated or failed to perform any of its representations, warranties, covenants or agreements set forth in the Merger Agreement which breach, violation or failure to perform would result in a failure of SIR's conditions to the consummation of the Merger and GOV does not cure such breach within a specified period; provided that SIR is not in breach of any of its representations, warranties, covenants or agreements set forth in the Merger Agreement which breach would result in a failure of GOV's conditions to the consummation of the Merger; or



•

if, prior to the receipt of the requisite approval of GOV shareholders, the GOV board of trustees (or an authorized committee thereof) (a) has made an adverse recommendation change, (b) fails to publicly reaffirm the GOV board of trustees' recommendation for the GOV Share Issuance Proposal within ten days of being requested to do so by SIR following the public announcement by any person of a competing acquisition proposal, (c) fails to include its recommendation for the GOV Share Issuance Proposal in this joint proxy statement/prospectus or (d) publicly announces its intention to do any of the foregoing; or



•

if, prior to the receipt of the requisite approval of SIR shareholders, the SIR board of trustees (or an authorized committee thereof) determines to enter into an acquisition agreement with respect to a superior proposal pursuant to the Merger Agreement.

Termination Fees and Expenses (See page 162)

        Neither party is entitled to any termination fee under the Merger Agreement. All fees and expenses incurred in connection with the Merger and the other Transactions will be paid by the party incurring those expenses, except that the parties will share equally any filing fees incurred in connection with the filing of the Form S-4, and, pursuant to the Registration Agreement, GOV is responsible for all of the costs and expenses incurred in connection with the Secondary Sale, including costs and expenses of SIR and its affiliates.

Material United States Federal Income Tax Consequences of the Merger (See page 139)

        The Merger is intended to qualify as a reorganization within the meaning of Section 368(a) of the IRC, and the consummation of the Merger is conditioned on the receipt by each of GOV and SIR of an opinion from their respective counsel that (i) the Merger will qualify as a reorganization within the meaning of Section 368(a) of the IRC and (ii) GOV and SIR will each be a party to that reorganization within the meaning of Section 368(b) of the IRC. Assuming the Merger qualifies as a reorganization within the meaning of Section 368(a) of the IRC, SIR shareholders generally will not recognize gain or loss for United States federal income tax purposes in connection with the Merger, except with respect to cash received in lieu of fractional GOV Common Shares. In addition, assuming the Merger so qualifies, SIR will not recognize any gain or loss as a result of the Merger. As SIR's successor, GOV will inherit SIR's tax attributes and would generally be liable for unpaid taxes, including penalties and interest (if any), of SIR.

Accounting Treatment of the Merger (See page 134)

        In accordance with United States generally accepted accounting principles, or GAAP, GOV will account for the Merger as a business combination with GOV treated as the acquirer of SIR. Under business combination accounting, the assets acquired and liabilities assumed will be recorded as of the acquisition date, at their respective fair value, and added to those of GOV. Any excess of purchase price over the fair values will be recorded as goodwill.

The GOV Special Meeting (See page 72)

        The GOV special meeting will be held at Two Newton Place, 255 Washington Street, Suite 100, Newton, Massachusetts 02458, at 1:00 p.m., local time, on December 20, 2018. You may vote at the GOV special meeting if you owned GOV Common Shares at the close of business on the GOV Record Date. On the GOV Record Date, there were 99,205,199 GOV Common Shares outstanding and entitled to vote at the GOV special meeting. You may cast one vote for each GOV Common Share that you owned on that date.

        At the GOV special meeting, GOV shareholders will be asked to consider and vote upon:

•

the GOV Share Issuance Proposal; and



•

the GOV Adjournment Proposal, if necessary or appropriate.

        The approval of the GOV Share Issuance Proposal is a condition to the completion of the Merger. If the GOV Share Issuance Proposal is not approved, the Merger and certain of the other Transactions will not be completed.

        The GOV Share Issuance Proposal and the GOV Adjournment Proposal require the affirmative vote of at least a majority of all the votes cast by holders of outstanding GOV Common Shares entitled to vote at the GOV special meeting on such proposals.

        As of the GOV Record Date, approximately 1.37% of the GOV Common Shares outstanding were beneficially owned by GOV's current trustees and executive officers and their affiliates. GOV currently expects that GOV's trustees and executive officers will vote their shares in favor of both of the proposals set forth above, although none has entered into any agreements obligating them to do so.

        The GOV board of trustees unanimously recommends that GOV shareholders vote "FOR" both of the proposals set forth above.

The SIR Special Meeting (See page 77)

        The SIR special meeting will be held at Two Newton Place, 255 Washington Street, Suite 100, Newton, Massachusetts 02458, at 3:00 p.m., local time, on December 20, 2018. You may vote at the SIR special meeting if you owned SIR Common Shares at the close of business on the SIR Record Date. On the SIR Record Date, there were 89,550,528 SIR Common Shares outstanding and entitled to vote at the SIR special meeting. Each SIR Common Share is entitled to cast one vote on all matters that come before the SIR special meeting.

        At the SIR special meeting, SIR shareholders will be asked to consider and vote upon:

•

the SIR Merger Proposal; and



•

the SIR Adjournment Proposal, if necessary or appropriate.

        The approval of the SIR Merger Proposal is a condition to the completion of the Merger. If the SIR Merger Proposal is not approved, the Merger and certain of the other Transactions will not be completed.

        Approval of the SIR Merger Proposal requires the affirmative vote of at least a majority of all the votes entitled to be cast by holders of outstanding SIR Common Shares at the SIR special meeting on such proposal. The approval of the SIR Adjournment Proposal requires the affirmative vote of at least a majority of all the votes cast by holders of outstanding SIR Common Shares entitled to vote at the SIR special meeting on such proposal.

        As of the SIR Record Date, approximately 1.92% of the SIR Common Shares outstanding were beneficially owned by SIR's current trustees and executive officers and their affiliates and approximately 27.8% of the SIR Common Shares outstanding were owned by GOV. SIR currently expects that SIR's trustees and executive officers will vote their shares in favor of the SIR Merger Proposal and the SIR Adjournment Proposal, although none has entered into any agreements obligating them to do so. Pursuant to the Merger Agreement, GOV must vote its SIR Common Shares in favor of the SIR Merger Proposal and the SIR Adjournment Proposal.

        The SIR board of trustees unanimously recommends that SIR shareholders vote "FOR" both of the proposals set forth above.

Comparison of Rights of GOV Shareholders and SIR Shareholders (See page 168)

        Upon consummation of the Merger, SIR shareholders will become GOV shareholders and their rights will therefore be governed by GOV's declaration of trust and the GOV Bylaws. GOV's declaration of trust and the GOV Bylaws are not materially different from SIR's declaration of trust and the SIR Bylaws.

Selected Historical Financial Information of GOV

        The following selected historical financial information for each of the years during the five year period ended December 31, 2017 and the selected balance sheet data as of December 31, 2017, 2016, 2015, 2014 and 2013 is unaudited and has been derived from GOV's audited consolidated financial statements contained in its Current Report on Form 8-K filed with the SEC on November 5, 2018, which is incorporated by reference into this joint proxy statement/prospectus. The selected historical financial information as of and for the nine months ended September 30, 2018 is unaudited and has been derived from GOV's unaudited condensed consolidated financial statements contained in GOV's Quarterly Report on Form 10-Q for the quarterly period ended September 30, 2018, which has been incorporated into this joint proxy statement/prospectus. Interim results for the nine months ended and as of September 30, 2018 are not necessarily indicative of, and are not projections for, the results to be expected for the fiscal year ending December 31, 2018 or of the combined company.

        The below information does not give effect to the Merger, the other Transactions or the Other Asset Dispositions.

        You should read this selected historical financial information together with the financial statements filed with the SEC and incorporated by reference into this joint proxy statement/prospectus and their accompanying notes and management's discussion and analysis of operations and financial condition of GOV.