Corporate Office Properties, L.P. Employee Retirement Savings Plan

Statements of Net Assets Available for Benefits

	December 31,
	2010			2009
Assets
Investments, at fair value
Mutual funds	$	19,833,593		$	14,197,694
Common/collective fund	656,585			1,002,549
Corporate Office Properties Trust common shares	1,231,135			1,053,532
Total investments, at fair value	21,721,313			16,253,775

Receivables
Notes receivable from participants	551,724			274,272
Employer contribution	21,593			27,664
Total receivables	573,317			301,936

Total assets	22,294,630			16,555,711

Adjustment to contract value
Adjustment from fair value to contract value for fully benefit-responsive investment contracts	(23,732		)	(30,106		)

Net assets available for benefits	$	22,270,898		$	16,525,605

See accompanying notes to financial statements.

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Corporate Office Properties, L.P. Employee Retirement Savings Plan

Statement of Changes in Net Assets Available for Benefits

Year Ended December 31, 2010

Additions
Investment income
Interest and dividend income from investments	$	453,102
Net appreciation in fair value of investments	2,414,417
Total investment income	2,867,519

Interest income on notes receivable from participants	22,641

Contributions
Employee	2,323,334
Employer	1,012,685
Rollovers	140,467
Total contributions	3,476,486

Total additions	6,366,646

Deductions
Benefits paid	618,302
Administrative expenses	3,051
Total deductions	621,353

Net increase	5,745,293

Net assets available for benefits
Beginning of year	16,525,605
End of year	$	22,270,898

See accompanying notes to financial statements.

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Corporate Office Properties, L.P. Employee Retirement Savings Plan

Notes to Financial Statements

1. Description of Plan

The following description of the Corporate Office Properties, L.P. Employee Retirement Savings Plan (the “Plan”) provides only general information. Participants should refer to the Plan document or summary plan description for a more complete description of the Plan’s provisions.

General

Corporate Office Properties, L.P. (the “Company”), which conducts almost all of Corporate Office Properties Trust’s operations and of which Corporate Office Properties Trust is the sole general partner, maintains the Plan for the benefit of the Company’s employees, as well as of those of its qualifying subsidiaries, who have completed at least 60 days of employment and are at least 21 years of age. Employees automatically enter the Plan as participants on the first day of the month that coincides with or next follows the date when they become eligible to enter the Plan unless they make an affirmative election to not participate. The Plan is a defined contribution pension plan intended to be qualified under section 401(a) of the Internal Revenue Code of 1986, as amended (the “IRC”), and is subject to the provisions of the Employee Retirement Income Security Act of 1974, as amended (“ERISA”). The Company serves as the Plan administrator and T. Rowe Price Trust Company is the Trustee for the Plan.

Contributions

Participants may contribute up to 90% of their compensation, as defined in the Plan, per pay period on a before-tax basis or after-tax basis, or a combination of both, subject to limitations under the IRC. Participants who are 50 years of age or older by the end of a particular plan year and have contributed the maximum 401(k) deferral amount allowed under the Plan for that year are eligible to contribute an additional portion of their annual compensation on a before-tax basis as catch-up contributions, up to the annual IRC limit. Participants may rollover amounts from traditional individual retirement accounts, 403(b) plans, 457 plans and other qualified retirement plans into the Plan. Participants direct the investment of their contributions into various investment options offered by the Plan. The Company matches 100% of the first 1% of pre-tax and/or after-tax contributions that participants contribute to the Plan and 50% of the next 5% in participant contributions to the Plan (representing an aggregate Company match of 3.5% on the first 6% of participant pre-tax and/or after-tax contributions to the Plan).

Participant Accounts

Each participant’s account is credited with the participant’s contributions, Company matching contributions and an allocation of Plan earnings (losses). Allocations are based on participant earnings or account balances, as defined in the Plan. The benefit to which a participant is entitled is the benefit that can be provided from the participant’s vested account.

Vesting

Participants immediately vest in their contributions and related earnings thereon. Vesting in the Company matching contribution portion of participant accounts is based on years of continuous service. For matching contributions made subsequent to December 31, 2008, a participant is 50% vested in Company matching contributions after one year of credited service and 100% vested after two years of credited service. For matching contributions made through December 31, 2008, a participant is 30% vested in Company matching contributions after one year of credited service, 60% vested after two years of credited service and 100% vested after three years of credited service.

Notes Receivable from Participants

Participants are eligible to obtain loans from the Plan, not to exceed the lesser of $50,000 or 50% of the vested balance of the participant’s account. The loans are secured by the balance in the participant’s account and bear interest at rates that are commensurate with local prevailing rates, as determined by the Plan administrator. At December 31, 2010, interest rates on participant loans ranged from 4.25% to 9.25% and the maturity dates on such loans ranged from January 2011 through November 2015. Repayment of participants’ loan principal and interest is obtained through bi-weekly payroll deductions from such participants. If participants revoke elections applicable to such payroll deductions, the entire unpaid principal sum of the participants’ loan plus accrued interest and any other amounts due under the loan will become due and payable. Partial or full early repayments of participant loans are permitted at any time.

Payment of Benefits

Upon termination of service, whether by death, disability, retirement or otherwise leaving the Company and its qualifying subsidiaries, a participant may elect to receive either a lump sum amount equal to the value of the participant’s

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vested interest in his or her account, or annual installments over a specified period unless such vested interest is $1,000 or less, in which case a distribution is made in a lump sum amount. Alternatively, a participant or applicable beneficiary may request that the Company make a direct transfer to another eligible retirement plan.

In the event of financial hardship (as defined by the Plan), participants may withdraw money from their Plan accounts while they are still employed. A participant cannot make elective deferral contributions to the Plan for six months after he or she takes a financial hardship distribution.

Forfeitures

Nonvested Company matching contributions are forfeited on the date a participant terminates employment with the Company or its qualifying subsidiaries. Forfeitures are available for the Company to apply against future Company contributions. Forfeited nonvested accounts totaled $0 at December 31, 2010 and $6,011 at December 31, 2009. The Plan used $15,139 in forfeited nonvested accounts during 2010 to reduce the Company’s funding requirements for additional employer contributions.

Investment Options

The Plan provides 23 T. Rowe Price mutual funds and one T. Rowe Price common/collective fund in which participants may choose to invest. Participants of the Plan may also choose to invest in additional mutual funds through a self-directed brokerage service provided by T. Rowe Price. In addition, the participants of the Plan may choose to invest in Corporate Office Properties Trust’s common shares.

2. Summary of Accounting Policies

Basis of Accounting

The financial statements of the Plan are prepared using the accrual basis of accounting.

Use of Estimates

The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and changes therein, and disclosure of contingent assets and liabilities. Actual results could differ from those estimates.

Payment of Benefits

Benefits are recorded when paid.

Investment Valuation and Income Recognition

The Plan’s investments are stated at fair value. Shares of mutual funds are valued at net asset value on the last business day of the Plan year. Shares in the T. Rowe Price Stable Asset Fund, which is a fully benefit-responsive common/collective fund, are valued at fair value with an adjustment to arrive at contract value. Contract value, which represents the contributions made under the contracts plus interest at the contract rates less withdrawals and administrative expenses, is the relevant measurement attribute for that portion of the net assets available for benefits because that is the amount participants would receive if they were to initiate permitted transactions under the terms of the Plan. Corporate Office Properties Trust’s common shares are valued at the closing market price of such shares at the end of the Plan year, as reported on the New York Stock Exchange.

Purchases and sales of securities are recorded on a trade-date basis. Dividends are recorded on the ex-dividend date. Interest income is recorded on the accrual basis.

The Plan presents in the Statement of Changes in Net Assets Available for Benefits the net appreciation in the fair value of its investments, which consists of the realized and unrealized gains or losses on those investments.

Administrative Expenses

Substantially all expenses incurred in connection with administration of the Plan are paid by the Company with the exception of loan fees, which are charged against the respective participants’ accounts.

Recent Accounting Pronouncements

In 2010, the Plan adopted an accounting standard issued by the Financial Accounting Standards Board (“FASB”) that affected its reporting for notes receivable from Plan participants. The standard requires that such notes be classified as notes receivable from participants and measured at their unpaid balance plus accrued but unpaid interest. Prior to adoption of this standard, notes receivable from participants were classified as investments and measured at fair value. The provisions of the

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standard were applied retrospectively to all periods presented, resulting in the reclassification of amounts reported for December 31, 2009. The standard affected how the Plan presents notes receivable from participants on the Statements of Net Assets Available for Benefits but did not otherwise have a material effect on the Plan’s financial statements.

In 2010, the Plan adopted guidance issued by the FASB that requires new disclosures and clarifications to existing disclosures pertaining to transfers in and out of Level 1 and Level 2 fair value measurements, presentation of activity within Level 3 fair value measurements and details of valuation techniques and inputs utilized. The Plan’s adoption of this guidance did not have a material effect on its financial statements or disclosures.

3. Investments, at fair value

The following presents the value and number of shares held of each investment that represents five percent or more of the Plan’s net assets as of the end of the respective years:

	Value of Investments at					Number of Shares Held
	December 31,					at December 31,
	2010			2009		2010		2009
Mutual funds:
T. Rowe Price Mid-Cap Growth Fund	$	2,200,198		$	1,625,722	37,591		34,233
T. Rowe Price Retirement 2020 Fund	1,924,852			1,295,820		117,083		88,755
T. Rowe Price Retirement 2030 Fund	1,686,331			1,191,429		97,589		78,798
T. Rowe Price Small-Cap Value Fund	1,532,284			882,582		42,410		29,938
T. Rowe Price Equity Index 500 Fund	1,395,303			1,109,967		41,208		36,962
T. Rowe Price Retirement 2035 Fund	1,390,123			925,600		113,665		86,911
T. Rowe Price Retirement 2025 Fund	1,378,402			984,276		114,485		92,769
T. Rowe Price Growth Stock Fund	1,324,915			1,011,004		41,210		36,750
T. Rowe Price Stable Value Fund			*	1,002,549			*	972,443
T. Rowe Price Equity Income Fund			*	948,132			*	45,171
Corporate Office Properties Trust common shares	1,231,135			1,053,532		35,226		28,761

* The balance of this investment does not represent more than 5% of the Plan’s assets at December 31, 2010.

The Plan’s investments appreciated in value by $2,414,417 in the year ended December 31, 2010 (including realized gains and losses on investments bought and sold and unrealized gains and losses on investments held during the year). This appreciation included $2,481,915 in net appreciation attributable to investments in mutual funds and a $67,498 net decrease attributable to investments in Corporate Office Properties Trust common shares.

Accounting standards define fair value as the exit price, or the amount that would be received upon sale of an asset or paid to transfer a liability in an orderly transaction between market participants as of the measurement date. The standards also establish a hierarchy for inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the most observable inputs be used when available. Observable inputs are inputs market participants would use in valuing the asset or liability developed based on market data obtained from sources independent of the Plan. Unobservable inputs are inputs that reflect assumptions about the factors market participants would use in valuing the asset or liability developed based upon the best information available given the circumstances. The hierarchy of these inputs is broken down into three levels: Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities; Level 2 inputs include (1) quoted prices for similar assets or liabilities in active markets, (2) quoted prices for identical or similar assets or liabilities in markets that are not active and (3) inputs (other than quoted prices) that are observable for the asset or liability, either directly or indirectly; and Level 3 inputs are unobservable inputs for the asset or liability. Categorization within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement.

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The table below sets forth the Plan’s assets measured at fair value on a recurring basis as of December 31, 2010 and 2009:

	Quoted Prices in
	Active Markets for		Significant Other		Significant
	Identical Assets		Observable Inputs		Unobservable Inputs
Description	(Level 1)		(Level 2)		(Level 3)		Total
December 31, 2010
Assets:
Mutual funds:
Stock funds	$	9,851,570	$	—	$	—	$	9,851,570
Retirement funds	9,276,188		—		—		9,276,188
Bond funds	705,835		—		—		705,835
Common/collective fund	—		656,585		—		656,585
Corporate Office Properties Trust common shares	1,231,135		—		—		1,231,135
Total assets	$	21,064,728	$	656,585	$	—	$	21,721,313
December 31, 2009
Assets:
Mutual funds:
Stock funds	$	7,373,424	$	—	$	—	$	7,373,424
Retirement funds	6,188,902		—		—		6,188,902
Bond funds	635,368		—		—		635,368
Common/collective fund	—		1,002,549		—		1,002,549
Corporate Office Properties Trust common shares	1,053,532		—		—		1,053,532
Total assets	$	15,251,226	$	1,002,549	$	—	$	16,253,775

The Plan’s Level 1 assets are valued at quoted market prices. The Plan’s Level 2 assets, representing investments in a common/collective fund, have underlying investments primarily in pools of investment contracts that are issued by insurance companies and commercial banks and contracts that are backed by high-quality bonds, bond and securities trusts and mutual funds; these investments are valued based on the aggregate market values of the applicable bonds, bond and securities trusts and other investments.

4. Plan Termination

Although it has not expressed any intention to do so, the Company has the right under the Plan to terminate the Plan and discontinue its contributions at any time, subject to the provisions of ERISA. In the event of termination, participants become 100% vested in their accounts.

5. Related Parties

Certain Plan investments are shares of mutual funds managed by T. Rowe Price Associates. T. Rowe Price Associates and T. Rowe Price Trust Company are subsidiaries of T. Rowe Price Group, Inc. Transactions with the Trustee, T. Rowe Price Trust Company, therefore qualify as party-in-interest transactions, which are exempt from the prohibited transaction rules of ERISA.

During 2010, the Plan purchased 9,899 common shares of Corporate Office Properties Trust for $373,925 and sold 3,434 common shares for $128,584. The Plan held 35,226 common shares valued at $1,231,135 at December 31, 2010 and 28,761 common shares valued at $1,053,532 at December 31, 2009.

6. Income Tax Status

The Plan administrator received a favorable determination letter dated July 13, 2005. The Plan administrator believes that the Plan is designed and operated in compliance with the applicable requirements of the IRC. As such, no provision for income taxes has been included in the Plan’s financial statements.

Accounting principles generally accepted in the United States of America require Plan management to evaluate tax positions taken by the Plan and recognize a tax liability (or asset) if the Plan has taken an uncertain position that more likely than not would not be sustained upon examination by the Internal Revenue Service. The Plan administrator has analyzed

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the tax positions by the Plan, and has concluded that as of December 31, 2010, there are no uncertain positions taken or expected to be taken that would require recognition of a liability (or asset) or disclosure in the financial statements. The Plan is subject to routine audits by taxing jurisdictions; however, there are currently no audits for any tax periods in progress. The Plan administrator believes it is no longer subject to income tax examinations for years prior to 2007.

7. Risks and Uncertainties

The Plan invests in various investment securities. Investment securities are exposed to various risks such as interest rate, market and credit risks. Due to the level of risk associated with certain investment securities, it is reasonably possible that changes in the values of investment securities will occur in the near term and that such changes could materially affect participants’ account balances and the amounts reported in the statement of net assets available for benefits.

8. Reconciliations of Financial Statements to Form 5500

The following is a reconciliation of net assets available for benefits per the financial statements to the Form 5500:

	December 31,
	2010		2009
Net assets available for benefits per the financial statements	$	22,270,898	$	16,525,605
Adjustment from fair value to contract value for fully benefit-responsive investment contracts	23,732		30,106
Net assets available for benefits per the Form 5500	$	22,294,630	$	16,555,711

The following is a reconciliation of income per the financial statements to the Form 5500 for the year ended December 31, 2010:

Total investment income	$	2,867,519
Interest income on notes receivable from participants	22,641
Adjustment from fair value to contract value for fully benefit-responsive investment contracts - current year	23,732
Adjustment from fair value to contract value for fully benefit-responsive investment contracts - prior year	(30,106		)
Total earnings on investments per the Form 5500	$	2,883,786

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Corporate Office Properties, L.P. Employee Retirement Savings Plan

Schedule H, Line 4i — Schedule of Assets (Held at End of Year)

December 31, 2010

		(c)
		Description of investment
	(b)	including maturity date, rate of		(e)
	Identity of issue, borrower, lessor	interest, collateral, par or	(d)	Current
(a)	or similar party	maturity value	Cost**	Value

*	T. Rowe Price Mid-Cap Growth Fund	Mutual fund		$	2,200,198
*	T. Rowe Price Retirement 2020 Fund	Mutual fund		1,924,852
*	T. Rowe Price Retirement 2030 Fund	Mutual fund		1,686,331
*	T. Rowe Price Small-Cap Value Fund	Mutual fund		1,532,284
*	T. Rowe Price Equity Index 500 Fund	Mutual fund		1,395,303
*	T. Rowe Price Retirement 2035 Fund	Mutual fund		1,390,123
*	T. Rowe Price Retirement 2025 Fund	Mutual fund		1,378,402
*	T. Rowe Price Growth Stock Fund	Mutual fund		1,324,915
*	T. Rowe Price Retirement 2040 Fund	Mutual fund		1,068,679
*	T. Rowe Price Equity Income Fund	Mutual fund		1,029,915
*	T. Rowe Price International Stock Fund	Mutual fund		937,978
*	T. Rowe Price Retirement 2045 Fund	Mutual fund		751,590
*	T. Rowe Price New Income Fund	Mutual fund		705,835
*	T. Rowe Price New Horizons Fund	Mutual fund		672,249
*	T. Rowe Price Balanced Fund	Mutual fund		604,361
*	T. Rowe Price Retirement Income Fund	Mutual fund		381,852
*	T. Rowe Price Retirement 2015 Fund	Mutual fund		258,550
*	T. Rowe Price Retirement 2050 Fund	Mutual fund		180,320
*	T. Rowe Price Retirement 2010 Fund	Mutual fund		137,191
*	T. Rowe Price Extended Equity Market Index	Mutual fund		126,792
*	T. Rowe Price Retirement 2005 Fund	Mutual fund		66,009
*	T. Rowe Price Retirement 2055 Fund	Mutual fund		52,289
*	T. Rowe Price Mid-Cap Value Fund	Mutual fund		27,575
	Registered investment companies			19,833,593
*	T. Rowe Price Stable Value Fund	Common/collective fund		656,585
*	Corporate Office Properties Trust common shares	Common shares		1,231,135
*	Notes receivable from participants	Interest rates ranging from 4.25% to 9.25%, maturity dates ranging from January 2011 through November 2015		551,724
				$	22,273,037

*		Denotes party-in-interest as defined by ERISA.
**		Cost information not required for participant-directed accounts.

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Report of Independent Registered Public Accounting Firm	3

Financial Statements
Statements of Net Assets Available for Benefits	4
Statement of Changes in Net Assets Available for Benefits	5
Notes to Financial Statements	6

Supplemental Schedules *
Schedule of Assets (Held at End of Year)	11

Signatures	12

Exhibit Index	13

Consent of Independent Registered Public Accounting Firm	Exhibit 23

	CORPORATE OFFICE PROPERTIES, L.P. EMPLOYEE
		RETIREMENT SAVINGS PLAN

		BY:	CORPORATE OFFICE PROPERTIES, L.P.,
			the Plan administrator

		BY:	CORPORATE OFFICE PROPERTIES TRUST,
			the sole general partner


Date: June 23, 2011		BY:	/s/ Randall M. Griffin
			Randall M. Griffin
			Chief Executive Officer

Exhibit Number		Exhibit Title
23		Consent of Independent Registered Public Accounting Firm