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;



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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