Prospectus Supplement No. 9		Filed Pursuant to Rule 424(b)(7)
To Prospectus Dated October 6, 2006		Registration No. 333-137894

CORPORATE OFFICE PROPERTIES TRUST

COMMON SHARES OF BENEFICIAL INTEREST

This prospectus supplement no. 9 supplements and amends the prospectus dated October 6, 2006, as previously supplemented on November 22, 2006, December 18, 2006, January 17, 2007, February 26, 2007, April 5, 2007, July 19, 2007, August 14, 2007 and September 25, 2007, relating to the resale from time to time by certain selling shareholders of our common shares of beneficial interest that may be issued in exchange for or on redemption of our 3.50% Exchangeable Senior Notes Due 2026.

This prospectus supplement should be read in conjunction with and accompanied by the prospectus and is qualified by reference to the prospectus, except to the extent that the information in this prospectus supplement supersedes the information contained in the prospectus.

The number of common shares issuable upon exchange of the notes shown in the table below assumes exchange of the full amount of notes held by each selling shareholder at the current conversion rate of 18.4908 common shares per $1,000 principal amount of notes. This conversion price is subject to adjustment in certain events. Accordingly, the number of conversion shares may increase or decrease from time to time. Information concerning other selling shareholders will be set forth in prospectus supplements from time to time, if required. The number of common shares owned by the other selling shareholders or any future transferee from any such holder assumes that they do not beneficially own any common shares other than the common shares that we may issue to them in exchange for or on redemption of the notes.

	Number of Shares Beneficially Owned		Percent of All Common Shares Beneficially Owned		Beneficial Ownership After Resale of Shares
Selling Shareholder	and Offered Hereby(1)		Before Resale(2)		Number of Shares	Percent

DBAG London +	596,790	(3)	1.3	%	—		*
JP Morgan Securities Inc. +	348,551			*	—		*
LLT Limited	7,636			*	—		*
Millenium Partners, L.P. +	138,681			*	—		*

* Indicates less than one percent (1%).

+ The selling shareholders identified with this symbol have identified that they are, or are affiliates of, registered broker-dealers. These selling shareholders have represented that they acquired their securities in the ordinary course of business and in the open market, and, at the time of the acquisition of the securities, had no agreements or understandings, directly or indirectly, with any person to distribute the securities. To the extent that we become aware that any such selling shareholder did not acquire its securities in the ordinary course of business or did have such an agreement or understanding, we will file a post-effective amendment to the registration statement of which this prospectus is a part to designate such person as an “underwriter” within the meaning of the Securities Act of 1933.

(1) Represents the maximum number of common shares issuable in exchange for or on redemption of all of the selling shareholder’s notes, based on the current conversion rate applicable to the notes of 18.4908 common shares per $1,000 principal amount of notes. This conversion rate is, however, subject to adjustment. As a result, the number of our common shares issuable upon conversion of the notes may increase or decrease in the future.

(2) Calculated based on 47,173,781 common shares outstanding as of August 31, 2007. In calculating this amount for each selling shareholder, we treated as outstanding the number of common shares in exchange for or on