Exhibit
10.1


TWENTY FIRST AMENDMENT TO


SECOND AMENDED AND RESTATED


LIMITED PARTNERSHIP AGREEMENT OF


CORPORATE
OFFICE PROPERTIES, L.P.


THIS TWENTY FIRST
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 July 20, 2006,
by and among the undersigned parties.


Recitals


A.   The Partnership is a limited partnership
organized under the Delaware Revised Uniform Limited Partnership Act (the “Act”)
and governed by that certain Second Amended and Restated Limited Partnership
Agreement dated as of January, 1999, as amended by that certain First Amendment
to Second Amended and Restated Limited Partnership Agreement dated as of  December 21, 1999, that certain Second
Amendment to Second Amended and Restated Limited Partnership Agreement dated as
of December 21, 1999, that certain Third Amendment to Second Amended and
Restated Limited Partnership Agreement dated as of  September 29, 2000, that certain Fourth
Amendment to Second Amended and Restated Limited Partnership Agreement dated as
of November 27, 2000, that certain Fifth Amendment to Second Amended and
Restated Limited Partnership Agreement dated as of January 25, 2001, that
certain Sixth Amendment to Second Amended and Restated Limited Partnership
Agreement dated as of April 6, 2001, that certain Seventh Amendment to the
Second Amended and Restated Partnership Agreement dated as of August 30, 2001,
that certain Eighth Amendment to the Second Amended and Restated Partnership
Agreement dated September 14, 2001, that certain Ninth Amendment to the Second
Amended and Restated Partnership Agreement dated October 16, 2001, that certain
Tenth Amendment to the Second Amended and Restated Partnership Agreement dated
December 29, 2001, that certain Eleventh Amendment to the Second Amended and
Restated Partnership Agreement dated December 15, 2002, that certain Twelfth
Amendment to the Second Amended and Restated Partnership Agreement dated June
2, 2003, that certain Thirteenth Amendment to the Second Amended and Restated
Partnership Agreement dated August 11, 2003, that certain Fourteenth Amendment
to the Second Amended and Restated Partnership Agreement dated December 18, 2003,
that certain Fifteenth Amendment to the Second Amended and Restated Partnership
Agreement dated  January 31, 2004, that
certain Sixteenth Amendment to the Second Amended and Restated Partnership
Agreement dated April 15, 2004, that certain Seventeenth Amendment to the
Second Amended and Restated Partnership Agreement dated September 23, 2004,
that certain Eighteenth Amendment to the Second Amended and Restated
Partnership Agreement dated April 18, 2005, that certain Nineteenth Amendment
to the Second Amended and Restated Partnership Agreement dated July 8, 2005 and
that certain Twentieth Amendment to the Second Amended and Restated Partnership
Agreement dated June 29, 2006 (as amended, the “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.   The General Partner has issued 3,390,000 of
its 7.625 % Series J Cumulative Redeemable Preferred Shares (the “Series J
Preferred REIT Shares”) in a public offering (the “Offering”).


D.   As required under Sections 4.2(B) and (C) of
the Agreement, the General Partner intends to transfer the net proceeds of the
Offering (or cause them to be transferred) to or for the benefit of the
Partnership in exchange for additional Partnership Interests in the Partnership
having designations, rights and preferences substantially similar to the
economic rights of the holders of the Series J Preferred REIT Shares (the “Series
J Preferred Units”).


E.    The General Partner desires to amend the
Agreement to acknowledge the contribution of the net proceeds of the Offering
by the General Partner to the Partnership in exchange for the Series J
Preferred Units.  Unless otherwise
defined herein, all capitalized terms used in this Amendment shall have the
same meanings as set forth in the Agreement.


NOW THEREFORE, in
consideration of the foregoing and of the mutual premises set forth herein, the
General Partner, intending to be legally bound hereby, hereby amends the
Agreement as follows, effective as of the date set forth above.


1.    The foregoing recitals to this Amendment are
hereby incorporated in and made a part of this Amendment.


(a)          Upon consummation of the Offering, the
General Partner shall contribute the net proceeds of the Offering to the
Partnership.


(b)         Upon the contribution of the net
proceeds of the Offering to the Partnership by the General Partner, and in
accordance with Section 4.2(B) of the Agreement, the Partnership shall issue to
the General Partner 3,390,000 Series J
Preferred Units, equal to the number of Series J Preferred REIT Shares issued
by the General Partner in connection with the Offering.


(c)           For purposes of the Agreement,
including the maintenance of Capital Accounts, the General Partner shall be
treated as making a Capital Contribution of $82,080,375, equal to the product
of $24.2125 times the number of Series J Preferred Units issued to the General
Partner.


(d)         The General Partner is hereby amending
Exhibit 1 to the Agreement by substituting for the existing addendum to Exhibit
1 the Addendum to



 2
 






 



Exhibit 1
in the form attached hereto to reflect the issuance of the Series J Preferred
Units to the General Partner.


2.             Except as explicitly modified by
this Amendment, all of the provisions of the Agreement are hereby ratified and
confirmed, and shall remain in full force and effect.


3.             This Amendment shall take effect
upon the contribution of the net proceeds of the Offering to the Partnership by
the General Partner, and in the event such contribution is not made, this
Amendment shall be of no force or effect.


(SIGNATURE
PAGE FOLLOWS)



 3
 






 



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/ Randall M. Griffin

  
	
   

  	
   

  	
  Name:

  	
   

  	
  Randall M. Griffin

  
	
   

  	
   

  	
  Title:

  	
   

  	
  President and Chief Executive Officer

  






 



 4
 






 



Exhibit 1 Addendum


 




	
  

  Series Preferred Units

  	
   

  	
  
Preferred Limited

  Partner

  	
   

  	
  

  No. of

  Preferred

  Units

  	
   

  	
  Liquidation Preference

  Per

  Preferred

  Unit

  	
   

  	
  

  Priority Percentage Return *

  	
   

  	
  

  Priority

  	
   

  	
  

  Conversion

  Factor

  	
   

  	
  

  Conversion

  Commencement

  Date

  
	
  F

  	
   

  	
  General
  Partner

  	
   

  	
  1,425,000

  	
   

  	
  $25

  	
   

  	
  10.25%

  	
   

  	
  Senior

  	
   

  	
  None

  	
   

  	
  N/A

  
	
  G

  	
   

  	
  General
  Partner

  	
   

  	
  2,200,000

  	
   

  	
  $25

  	
   

  	
  8%

  	
   

  	
  Senior

  	
   

  	
  None

  	
   

  	
  N/A

  
	
  H

  	
   

  	
  General
  Partner

  	
   

  	
  2,000,000

  	
   

  	
  $25

  	
   

  	
  7.5%

  	
   

  	
  Senior

  	
   

  	
  None

  	
   

  	
  N/A

  
	
  I

  	
   

  	
  TRC
  Associates Limited Partnership

  	
   

  	
  352,000

  	
   

  	
  **

  	
   

  	
  ***    

  	
   

  	
  Senior

  	
   

  	
  .05/1

  	
   

  	
  September
  23, 2004

  
	
  J

  	
   

  	
  General
  Partner

  	
   

  	
  3,390,000

  	
   

  	
  $25

  	
   

  	
  7.625%

  	
   

  	
  Senior

  	
   

  	
  None

  	
   

  	
  N/A

  










*                    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.”


**             Liquidation
Preference Per Series I Preferred Unit shall equal $25.00 plus all accrued and
unpaid distributions thereon.  In
determining the Liquidation Preference, unpaid distributions shall accrue and
be compounded on a quarterly basis.


***      Priority
Return Percentage for the Series I Preferred Units shall be governed by Section
4 of the Seventeenth Amendment.  The
Distribution Period for the Series I Preferred Units shall be each calendar
quarter ending on March 31, June 30, September 30 and December 31, of each
year.


 



 5