EXHIBIT 5.1




                                       February 5, 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-4 (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 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 



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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 and
delivered in exchange for shares of Common Stock in accordance with the terms of
Merger Agreement, 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, 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





                                       February 5, 1998

Corporate Office Properties Trust
One Logan Square, Suite 1105
Philadelphia, Pennsylvania 19103

               Re:  Corporate Office Properties Trust
                    Registration Statement on Form S-4


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 
2,341,083 common shares (the "Shares") of beneficial interest, $.01 par value 
per share, of the Company ("Common Shares"), to be issued by the Company in 
connection with the mergers (the "Mergers") of (a) Corporate Office 
Properties Trust, Inc., a Minnesota corporation (the "Minnesota 
Corporation"), with and into COPT, Inc., a Maryland corporation and a 
wholly-owned subsidiary of the Minnesota Corporation (the "Maryland 
Corporation"), and (b) the Maryland Corporation with and into the Company, 
both pursuant to the Agreement and Plan of Merger, dated as of January 31, 1998
(the "Merger Agreement"), by and among the Company, the Minnesota Corporation 
and the Maryland Corporation, 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") on February 5, 
1998, including the related form



Corporate Office Properties Trust
February 5, 1998
Page 2

of Proxy Statement and Prospectus (the "Proxy Statement/Prospectus") included 
therein;

     2.  The 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 its 
Secretary;

     4.  Resolutions adopted by the Board of Trustees, or a duly authorized 
committee thereof, of the Company relating to (i) the authorization, sale, 
issuance and registration of the Shares and (ii) the approval of the Merger 
Agreement, certified as of a recent date by the Secretary of the Company;

     5.  The Merger Agreement;

     6.  A certificate of the SDAT, as of a recent date, as to the good 
standing of the Company;

     7.  A certificate executed by an officer of the Company, dated the date 
hereof; 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.



Corporate Office Properties Trust
February 5, 1998
Page 3


     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 in the Documents are true and complete. There are no 
oral or written modifications or amendments to the Documents, by action or 
omission of the parties or otherwise.

     5.  All shares of stock of the Minnesota Corporation issued and 
outstanding immediately prior to the Merger are duly authorized, validly 
issued, fully paid and non-assessable.

     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 and validly authorized and, when and if 
issued in accordance with the resolutions of the Board of Trustees of the 
Company authorizing their issuance and with the Merger Agreement, 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.



Corporate Office Properties Trust
February 5, 1998
Page 4



      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.

      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,