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Exhibit 10.1

Twelfth Amendment
To
Second Amended and Restated
Limited Partnership Agreement
Of
Corporate Office Properties, L.P.

        This Twelfth Amendment (the "Amendment") to the Second Amended and
Restated Limited Partnership Agreement Of Corporate Office Properties, L.P., a
Delaware limited partnership (the "Partnership"), is made and entered into as of
June 2, 2003, by the undersigned.

Recitals

        A.    The Partnership is a limited partnership organized under the
Delaware Revised Uniform Limited Partnership Act and governed by that certain
Second Amended and Restated Limited Partnership Agreement dated as of
December 7, 1999 (the "Partnership Agreement").

        B.    The sole general partner of the Partnership is Corporate Office
Properties Trust, a real estate investment trust formed under the laws of the
State of Maryland (the "General Partner").

        C.    Pursuant to Section 11.1(B) (iii), the General Partner desires to
amend the Partnership Agreement to reflect the admission, substitution,
termination and/or withdrawal of various limited partners in accordance with the
terms of the Partnership Agreement.

        NOW THEREFORE, the General Partner, intending to be legally bound,
hereby amends the Partnership Agreement as follows, effective as of the date
first set forth above.

        1.     Exhibit 1, Schedule of Partners, as attached hereto and by this
reference made a part hereof, is hereby substituted for and intended to replace
any prior Exhibit 1 attached to a prior Amendment to the Partnership Agreement,
and as attached hereto shall be a full and complete listing of all the general
and limited partners of the Partnership as of the date of this Amendment, same
being intended and hereby superceding all prior Exhibit 1 listings.

        In Witness Whereof the General Partner has executed this Amendment as of
the day and year first above written.

    Corporate Office Properties Trust, a
Maryland Real Estate Investment Trust
 
 
By:
 
/s/  ROGER A. WAESCHE, JR.      

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Roger A. Waesche, Jr.
Senior Vice President

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Schedule of Partners

General Partner

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  Partnership Units

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Corporate Office Properties Trust   27,082,879
Limited Partners and Preferred Limited Partners

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  Jay H. Shidler   452,878 Shidler Equities, L.P.   2,995,439 Clay W. Hamlin,
III   568,492 LBCW Limited Partnership   3,161,427 Robert L. Denton   434,910
James K. Davis   51,589 John E. De B. Blockey, Trustee of the John E. de B.
Blockey Living Trust dated 9/12/88   300,625 Henry D. Bullock   76,533 Frederick
K. Ito Trust   29,140 June Y. I. Ito Trust   29,135 RP Investments, LLC  
268,671 Denise J. Liszewski   34,333 Samuel Tang   22,889 Lawrence J. Taff  
13,733 Kimberly F. Acquino   5,874 M.O.R. XXIX Associates Limited Partnership  
148,381 M.O.R. 44 Gateway Associates Limited Partnership   1 John Parsinen  
90,000 M.O.R. Commons Limited Partnership   7 John Edward De Burgh Blockey and
Sanda Juanita Blockey   50,476 Anthony Muscatello   90,905 Lynn Hamlin   121,411
    36,029,748    

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EXHIBIT 1

Addendum

Series
Preferred
Units

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  Preferred
Limited Partner

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  No. of
Preferred
Units

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  Liquidation
Preference
Per Preferred
Unit

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  Priority
Percentage
Return*

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  Priority

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  Conversion
Factor

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  Conversion
Commencement
Date

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B   General Partner   1,250,000   $ 25   2.50 % Senior   None   N/A C   UPG  
1,016,662   $ 25   **   Senior   2.381   12/22/2000 D   General Partner  
544,000   $ 25   1.00 % Senior   ***   *** E   General Partner   1,150,000   $
25   2.5625 % Senior   None   N/A F   General Partner   1,425,000   $ 25      
Senior   None   N/A

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*Priority Return Percentage is expressed as a percentage of the Liquidation
Preference per Distribution Period. See the Agreement for the definitions of
"Priority Return Percentage," "Liquidation Preference" and "Distribution
Period."

**Priority Percentage Return for the Series C Preferred Units shall be:

2.25% from December 21, 1999 to December 20, 2009; 2.625% from December 21, 2009
to December 20, 2014; and 3.00% thereafter.

The Distribution Period for the Series C Preferred Units shall be each calendar
quarter ending March 31, June 30, September 30 and December 31 of each year.

***With respect to any series of Preferred Units issued to the General Partner
pursuant to Section 4.2(B) of the Agreement, the Conversion Commencement Date
and the applicable Conversion Factor shall correspond to the conversion
commencement date and conversion factor of the related issuance of securities by
the General Partner as provided in Section 4.2(B) of the Agreement. See
Section 9.8(A)(1) of the Agreement.

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QuickLinks

Exhibit 10.1

Twelfth Amendment To Second Amended and Restated Limited Partnership Agreement
Of Corporate Office Properties, L.P.
Schedule of Partners
EXHIBIT 1 Addendum