Schedule 13G

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No. n/a )*

Corporate Office Properties Trust

(Name of Issuer)

Common Shares of Beneficial Interest

(Title of Class of Securities)

22002T108

(CUSIP Number)

December 31, 2009

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

x Rule 13d-1(b)

o Rule 13d-1(c)

o Rule 13d-1(d)

* The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

Item 1.

(a)

Name of Issuer

Corporate Office Properties Trust

(b)

Address of Issuer’s Principal Executive Offices

6711 Columbia Gateway Drive
Suite 300
Columbia, MD 21046

Item 2.

(a)

Name of Person Filing

Heitman Real Estate Securities LLC

(b)

Address of Principal Business Office or, if none, Residence

191 North Wacker Drive
Suite 2500
Chicago, Illinois 60606

(c)

Citizenship

United States

(d)

Title of Class of Securities

Common Shares of Beneficial Interest

(e)

CUSIP Number

22002T108

Item 3.

If this statement is filed pursuant to §§240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:

(a)

Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o).

(b)

Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c).

(c)

Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. o78c).

(d)

Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C 80a-8).

(e)

An investment adviser in accordance with §240.13d-1(b)(1)(ii)(E);

(f)

An employee benefit plan or endowment fund in accordance with §240.13d-1(b)(1)(ii)(F);

(g)

A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);

(h)

A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);

(i)

A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);

(j)

Group, in accordance with §240.13d-1(b)(1)(ii)(J).

Item 4.

Ownership.

Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1.

(a)

Amount beneficially owned: 3,885,398

(b)

Percent of class: 6.67%

(c)

Number of shares as to which the person has:

(i)

Sole power to vote or to direct the vote: 2,842,324

(ii)

Shared power to vote or to direct the vote: 0

(iii)

Sole power to dispose or to direct the disposition of: 3,885,398

(iv)

Shared power to dispose or to direct the disposition of: 0

Item 5.

Ownership of Five Percent or Less of a Class

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following o .

Not applicable.

Item 6.

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

Heitman Real Estate Securities LLC serves as sub-investment adviser to the Old Mutual Advisor Funds II Old Mutual Heitman REIT Fund, the Penn Series Funds, Inc. REIT Fund, the Russell Investment Funds Real Estate Securities Fund and the Russell Investment Company Real Estate Securities Fund, all registered investment companies, and as investment adviser to 1,545 separate account clients.

The Old Mutual Advisor Funds II Old Mutual Heitman REIT Fund, the Penn Series Funds, Inc. REIT Fund, the Russell Investment Funds Real Estate Securities Fund and the Russell Investment Company Real Estate Securities Fund and 1,545 separate account clients have given dispositive power to Heitman Real Estate Securities LLC the right to receive or the power to direct the receipt of dividends from, or proceeds from the sale of 3,885,398 shares, 6.67% of this issuer.

Item 7.

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

Not applicable.

Item 8.

Identification and Classification of Members of the Group

Not applicable.

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 and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.


1	NAMES OF REPORTING PERSONS I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY)
	HEITMAN REAL ESTATE SECURITIES LLC 36-4265577

2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)
	(a) o
	(b) x

3	SEC USE ONLY



4	CITIZENSHIP OR PLACE OF ORGANIZATION

	Delaware

NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:		5	SOLE VOTING POWER

			2,842,324

		6	SHARED VOTING POWER

			0

		7	SOLE DISPOSITIVE POWER

			3,885,398

		8	SHARED DISPOSITIVE POWER

			0

9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

	3,885,398

10	CHECK IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS)

	o

11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)



12	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)

	IA
	FOOTNOTES



Date: February 12, 2010	By:	/s/ Nancy B. Lynn
		Name: Nancy B. Lynn
		Title: Vice President