EXHIBIT 5.1 [Letterhead of Cahill Gordon & Reindel] April 9, 1998 Corporate Office Properties Trust One Logan Square, Suite 1105 Philadelphia, Pennsylvania 19103 Ladies and Gentlemen: We have acted as counsel for Corporate Office Properties Trust (the "Trust") in connection with the Registration Statement on Form S-11 (the "Registration Statement") filed by the Trust with the Securities and Exchange Commission (the "Commission") for registration under the Securities Act of 1933, as amended (the "Securities Act"), of up to 8,625,000 common shares of beneficial interest, par value $0.01 per share (the "Common Shares"). Capitalized terms used and not otherwise defined herein shall have the meanings ascribed to such terms in the Registration Statement. In connection therewith, we have examined, among other things, originals or copies, certified or otherwise identified to our satisfaction, of the Declaration of Trust and Bylaws of the Trust, resolutions of the Board of Trustees of the Trust with respect to the filing of the Registration Statement and such other documents as we have deemed necessary or appropriate for the purpose of rendering this opinion. In our examination of documents, instruments and other papers, we have assumed the genuineness of all signatures on original and certified documents and the conformity to original and certified documents of all copies submitted to us as conformed, photostatic or other copies. As to matters of fact, we have relied upon representations of officers of the Trust. Based upon the foregoing examination, information supplied and assumptions, it is our opinion that the Common Shares have been duly authorized by all necessary action of the Trust and when the Common Shares have been issued, delivered and paid for as described in the Registration Statement, such Common Shares will be validly issued, fully paid and non-assessable. We are attorneys admitted to practice in the State of New York. We express no opinion concerning the laws of any jurisdiction other than the laws of the United States of America and the laws of the State of New York. With respect to matters of Maryland law, we have relied, without independent investigation, upon the opinion of Ballard Spahr Andrews & Ingersoll, LLP, a copy of which is attached hereto. We hereby consent to the reference to our firm in the Registration Statement under the caption "Legal Matters" and to the inclusion of this opinion as an exhibit to the Registration Statement. Our consent to such reference does not constitute a consent under Section 7 of the Securities Act as in consenting to such reference we have not certified any part of the Registration Statement and do not otherwise come within the categories of persons whose consent is required under Section 7 or under the rules and regulations of the Commission thereunder. Very truly yours, /s/ Cahill Gordon & Reindel [Letterhead of Ballard Spahr Andrews & Ingersoll, LLP] April 9, 1998 Corporate Office Properties Trust One Logan Square, Suite 1105 Philadelphia, Pennsylvania 19103 Re: Corporate Office Properties Trust Registration Statement on Form S-11 (Registration No. 333-47465) ------------------------------------ Ladies and Gentlemen: We have served as Maryland counsel to Corporate Office Properties Trust, a Maryland real estate investment trust (the "Company"), in connection with certain matters of Maryland law arising out of the registration of up to 8,625,000 common shares (the "Shares") of beneficial interest, $.01 par value per share, of the Company ("Common Shares") (including an option to purchase up to an additional 1,125,000 Shares), as described in the above-referenced Registration Statement, under the Securities Act of 1933, as amended (the "1933 Act"). Capitalized terms used but not defined herein shall have the meanings given to them in the Registration Statement. In connection with our representation of the Company, and as a basis for the opinion hereinafter set forth, we have examined originals, or copies certified or otherwise identified to our satisfaction, of the following documents (hereinafter collectively referred to as the "Documents"): 1. The Registration Statement in the form in which it was transmitted to the Securities and Exchange Commission (the "Commission"), including the related form of prospectus in the form in which it was transmitted to the Commission under the 1933 Act; 2. The Amended and Restated Declaration of Trust of the Company, certified as of a recent date by the State Department of Assessments and Taxation of Maryland (the "SDAT"); 3. The Bylaws of the Company, certified as of a recent date by an officer of the Company; Corporate Office Properties Trust April , 1998 Page 2 4. Resolutions adopted by the Board of Trustees, or a duly authorized committee thereof, of the Company relating to the authorization, sale, issuance and registration of the Shares (the "Resolutions"), certified as of a recent date by an officer of the Company; 5. A certificate of the SDAT, as of a recent date, as to the good standing of the Company; 6. A certificate executed by an officer of the Company, dated the date hereof; 7. A form of certificate evidencing a Common Share, certified as of a recent date by an officer of the Company; and 8. Such other documents and matters as we have deemed necessary or appropriate to express the opinion set forth in this letter, subject to the assumptions, limitations and qualifications stated herein. In expressing the opinion set forth below, we have assumed, and so far as is known to us there are no facts inconsistent with, the following: 1. Each of the parties (other than the Company) executing any of the Documents has duly and validly executed and delivered each of the Documents to which such party is a signatory, and such party's obligations set forth therein are legal, valid and binding and are enforceable in accordance with all stated terms. 2. Each individual executing any of the Documents on behalf of a party (other than the Company) is duly authorized to do so. 3. Each individual executing any of the Documents, whether on behalf of such individual or another person, is legally competent to do so. 4. All Documents submitted to us as originals are authentic. The form and content of the Documents submitted to us as unexecuted drafts do not differ in any respect relevant to this opinion from the form and content of such Documents as executed and delivered. All Documents submitted to us as certified or photostatic copies conform to the original documents. All signatures on all such Documents are genuine. All public records reviewed or relied upon by us or on our behalf are true and complete. All statements and information contained Corporate Office Properties Trust April __, 1998 Page 3 in the Documents are true and complete. There has been no oral or written modification or amendment to any of the Documents, and there has been no waiver of any provision of any of the Documents, by action or omission of the parties or otherwise. The phrase "known to us" is limited to the actual knowledge, without independent inquiry, of the lawyers at our firm who have performed legal services in connection with the issuance of this opinion. Based upon the foregoing, and subject to the assumptions, limitations and qualifications stated herein, it is our opinion that: 1. The Company is a real estate investment trust duly formed and existing under and by virtue of the laws of the State of Maryland and is in good standing with the SDAT. 2. The Shares have been duly authorized and, when and if issued in accordance with the Resolutions, will be duly and validly issued, fully paid and nonassessable. The foregoing opinion is limited to the substantive laws of the State of Maryland and we do not express any opinion herein concerning any other law. We express no opinion as to compliance with the securities (or "blue sky") laws of the State of Maryland. We assume no obligation to supplement this opinion if any applicable law changes after the date hereof or if we become aware of any fact that might change the opinion expressed herein after the date hereof. This opinion is being furnished to you for your submission to the Commission as an exhibit to the Registration Statement and, accordingly, may not be relied upon by, quoted in any manner to, or delivered to any other person or entity (other than Cahill Gordon & Reindel, counsel to the Company) without, in each instance, our prior written consent. Corporate Office Properties Trust April __, 1998 Page 4 We hereby consent to the filing of this opinion as an exhibit to the Registration Statement and to the use of the name of our firm in the section entitled "Legal Matters" in the Registration Statement. In giving this consent, we do not admit that we are within the category of persons whose consent is required by Section 7 of the 1933 Act. Very truly yours, Ballard Spahr Andrews & Ingersoll, LLP