SCHEDULE 13G

Amendment No. 4
Corporate Office Properties Trust
Common Stock
Cusip #22002T108


Cusip #22002T108
Item 1:	Reporting Person - FMR LLC
Item 4:	Delaware
Item 5:	565,012
Item 6:	0
Item 7:	3,744,469
Item 8:	0
Item 9:	3,744,469
Item 11:	6.516%
Item 12:	    HC


Cusip #22002T108
Item 1:	Reporting Person - Edward C. Johnson 3d
Item 4:	United States of America
Item 5:	0
Item 6:	0
Item 7:	3,744,469
Item 8:	0
Item 9:	3,744,469
Item 11:	6.516%
Item 12:	IN




	SCHEDULE 13G - TO BE INCLUDED IN
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)



Item 1(a).	Name of Issuer:

		Corporate Office Properties Trust

Item 1(b).	Name of Issuer's Principal Executive Offices:

		6711 Columbia Gateway Drive
		Suite 300
		Columbia, Maryland  21046


Item 2(a).	Name of Person Filing:

		FMR LLC

Item 2(b).	Address or Principal Business Office or, if None,
Residence:

		82 Devonshire Street, Boston,
Massachusetts  02109

Item 2(c).	Citizenship:

		Not applicable

Item 2(d).	Title of Class of Securities:

		Common Stock

Item 2(e).	CUSIP Number:

		22002T108

Item 3.	This statement is filed pursuant to Rule 13d-1(b) or 13d-2(b)
and the person filing, FMR LLC, is a parent holding company
in accordance with Section 240.13d-1(b)(ii)(G).  (Note:  See
Item 7).

Item 4.	Ownership

	(a)	Amount Beneficially Owned:	3,744,469

	(b)	Percent of Class:	6.516%

	(c)	Number of shares as to which such
person has:

	(i)	sole power to vote or to direct
the vote:	565,012

	(ii)	shared power to vote or to
direct the vote:	0

	(iii)	sole power to dispose or to
direct the disposition of:	3,744,469

	(iv)	shared power to dispose or to
direct the disposition of:	0



Item 5.	Ownership of Five Percent or Less of a Class.

	Not applicable.

Item 6.	Ownership of More than Five Percent on Behalf of Another
Person.

	Various persons have the right to receive or the power to
direct the receipt of dividends from, or the proceeds from the
sale of, the Common Stock of Corporate Office Properties
Trust.  No one person's interest in the Common Stock of
Corporate Office Properties Trust is more than five percent of
the total outstanding Common Stock.

Item 7.	Identification and Classification of the Subsidiary Which
Acquired the Security Being Reported on By the Parent
Holding Company.

	See attached Exhibit A.

Item 8.	Identification and Classification of Members of
the Group.

	Not applicable.  See attached Exhibit A.

Item 9.	Notice of Dissolution of Group.

	Not applicable.

Item 10.	Certification.

	By signing below I certify that, to the best of my knowledge
and belief, the securities referred to above were acquired in the
ordinary course of business and were not acquired for the
purpose of and do not have the effect of changing or
influencing the control of the issuer of such securities and
were not acquired in connection with or as a participant in any
transaction having such purpose or effect.

Signature

	After reasonable inquiry and to the best of my knowledge and
belief, I certify that the information set forth in this statement
is true, complete and correct.

June 09, 2009
Date

 /s/ Scott C. Goebel
Signature

Scott C. Goebel
Duly authorized under Power of Attorney
effective as of June 1, 2008 by and on behalf of FMR LLC
and its direct and indirect  subsidiaries


	SCHEDULE 13G - TO BE INCLUDED IN
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)



	Pursuant to the instructions in Item 7 of Schedule 13G,
Fidelity Management & Research Company ("Fidelity"), 82
Devonshire Street, Boston, Massachusetts 02109, a wholly-
owned subsidiary of FMR LLC and an investment adviser
registered under Section 203 of the Investment Advisers Act
of 1940, is the beneficial owner of 3,174,677 shares or 5.524%
of the Common Stock outstanding of Corporate Office
Properties Trust  ("the Company") as a result of acting as
investment adviser to various investment companies registered
under Section 8 of the Investment Company Act of 1940. The
number of shares of Common Stock of Corporate Office
Properties Trust owned by the investment companies at May
31, 2009 included 66,810 shares of Common Stock resulting
from the assumed conversion of $3,600,000 principal amount
of CORPORATE OF CV 3.5% 9/26 (18.5583 shares of
Common Stock for each $1,000 principal amount of
debenture).

	Edward C. Johnson 3d and FMR LLC, through its
control of Fidelity, and the funds each has sole power to
dispose of the 3,174,677 shares owned by the Funds.

	Members of the family of Edward C. Johnson 3d,
Chairman of FMR LLC, are the predominant owners, directly
or through trusts, of Series B voting common shares of FMR
LLC, representing 49% of the voting power of FMR LLC.
The Johnson family group and all other Series B shareholders
have entered into a shareholders' voting agreement under
which all Series B voting common shares will be voted in
accordance with the majority vote of Series B voting common
shares.  Accordingly, through their ownership of voting
common shares and the execution of the shareholders' voting
agreement, members of the Johnson family may be deemed,
under the Investment Company Act of 1940, to form a
controlling group with respect to FMR LLC.

	Neither FMR LLC nor Edward C. Johnson 3d,
Chairman of FMR LLC, has the sole power to vote or direct
the voting of the shares owned directly by the Fidelity Funds,
which power resides with the Funds' Boards of Trustees.
Fidelity carries out the voting of the shares under written
guidelines established by the Funds' Boards of Trustees.

	Pyramis Global Advisors, LLC ("PGALLC"), 900
Salem Street, Smithfield, Rhode Island, 02917, an indirect
wholly-owned subsidiary of FMR LLC and an investment
adviser registered under Section 203 of the Investment
Advisers Act of 1940, is the beneficial owner of 83,310 shares
or 0.145% of the outstanding Common Stock of Corporate
Office Properties Trust as a result of its serving as investment
adviser to institutional accounts, non-U.S. mutual funds, or
investment companies registered under Section 8 of the
Investment Company Act of 1940 owning such shares.

	Edward C. Johnson 3d and FMR LLC, through its
control of PGALLC, each has sole dispositive power over
83,310 shares and sole power to vote or to direct the voting of
83,310 shares of Common Stock owned by the institutional
accounts or funds advised by PGALLC as reported above.

	Pyramis Global Advisors Trust Company ("PGATC"),
900 Salem Street, Smithfield, Rhode Island, 02917, an indirect
wholly-owned subsidiary of FMR LLC and a bank as defined
in Section 3(a)(6) of the Securities Exchange Act of 1934, is
the beneficial owner of 340,082 shares or 0.592% of the
outstanding Common Stock of the Corporate Office Properties
Trust as a result of its serving as investment manager of
institutional accounts owning such shares. The number of
shares of Common Stock of Corporate Office Properties Trust
owned by the institutional account(s) at May 31, 2009
included 25,982 shares of Common Stock resulting from the
assumed conversion of $1,400,000 principal amount of
CORPORATE OF CV 3.5% 9/26 (18.5583 shares of Common
Stock for each $1,000 principal amount of debenture).

	Edward C. Johnson 3d and FMR LLC, through its
control of Pyramis Global Advisors Trust Company, each has
sole dispositive power over 340,082 shares and sole power to
vote or to direct the voting of 340,082 shares of Common
Stock owned by the institutional accounts managed by
PGATC as reported above.

	FIL Limited ("FIL"), Pembroke Hall, 42 Crow Lane,
Hamilton, Bermuda, and various foreign-based subsidiaries
provide investment advisory and management services to a
number of non-U.S. investment companies and certain
institutional investors.  FIL, which is a qualified institution
under section 240.13d-1(b)(1) pursuant to an SEC No-Action
letter dated October 5, 2000, is the beneficial owner of
146,400 shares or 0.255% of the Common Stock outstanding
of the Company.

	Partnerships controlled predominantly by members of
the family of Edward C. Johnson 3d, Chairman of FMR LLC
and FIL, or trusts for their benefit, own shares of FIL voting
stock with the right to cast approximately 47% of the total
votes which may be cast by all holders of FIL voting stock.
FMR LLC and FIL are separate and independent corporate
entities, and their Boards of Directors are generally composed
of different individuals.

	FMR LLC and FIL are of the view that they are not
acting as a "group" for purposes of Section 13(d) under the
Securities Exchange Act of 1934 (the "1934" Act) and that
they are not otherwise required to attribute to each other the
"beneficial ownership" of securities "beneficially owned" by
the other corporation within the meaning of Rule 13d-3
promulgated under the 1934 Act.  Therefore, they are of the
view that the shares held by the other corporation need not be
aggregated for purposes of Section 13(d). However, FMR
LLC is making this filing on a voluntary basis as if all of the
shares are beneficially owned by FMR LLC and FIL on a joint
basis.

	FIL has sole dispositive power over 146,400 shares
owned by the International Funds. FIL has sole power to vote
or direct the voting of 140,500 shares and no power to vote or
direct the voting of 5,900 shares of Common Stock held by the
International Funds as reported above.


	SCHEDULE 13G - TO BE INCLUDED IN
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)
	RULE 13d-1(f)(1)  AGREEMENT

	The undersigned persons, on June 09, 2009, agree and
consent to the joint filing on their behalf of this Schedule 13G
in connection with their beneficial ownership of the Common
Stock of Corporate Office Properties Trust at May 31, 2009.

	FMR LLC

	By /s/ Scott C. Goebel
	Scott C. Goebel
	Duly authorized under Power of Attorney effective as
of June 1, 2008, by and on behalf of FMR LLC and its direct
and indirect subsidiaries

	Edward C. Johnson 3d

	By /s/ Scott C. Goebel
	Scott C. Goebel
	Duly authorized under Power of Attorney effective as
of June 1, 2008, by and on behalf of Edward C. Johnson 3d

	Fidelity Management & Research Company

	By /s/ Scott C. Goebel
	Scott C. Goebel
	Senior V.P. and General Counsel