N-CSRS

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM N-CSR

CERTIFIED SHAREHOLDER REPORT OF REGISTERED MANAGEMENT

INVESTMENT COMPANIES

Investment Company Act file number 811-09333

Oppenheimer Main Street Mid Cap Fund

(Exact name of registrant as specified in charter)

6803 South Tucson Way, Centennial, Colorado 80112-3924

(Address of principal executive offices) (Zip code)

Arthur S. Gabinet

OFI Global Asset Management, Inc.

225 Liberty Street, New York, New York 10281-1008

(Name and address of agent for service)

Registrant’s telephone number, including area code: (303) 768-3200

Date of fiscal year end: June 30

Date of reporting period: 12/31/2014

Item 1. Reports to Stockholders.

Table of Contents

Fund Performance Discussion	3
Top Holdings and Allocations	6
Fund Expenses	9
Statement of Investments	11
Statement of Assets and Liabilities	14
Statement of Operations	16
Statements of Changes in Net Assets	17
Financial Highlights	18
Notes to Financial Statements	25
Board Approval of the Fund’s Investment Advisory and Sub-Advisory Agreements	36
Portfolio Proxy Voting Policies and Procedures; Updates to Statement of Investments	39
Trustees and Officers	40
Privacy Policy Notice	41

Class A Shares

AVERAGE ANNUAL TOTAL RETURNS AT 12/31/14

	Class A Shares of the Fund
	Without Sales Charge		With Sales Charge		Russell 2500 Index		Russell Midcap Index
6-Month		3.31%		-2.64%		1.06%		4.18%
1-Year		12.27		5.82		7.07		13.22
5-Year		15.96		14.59		16.36		17.19
10-Year		8.20		7.57		8.72		9.56

Prior to June 30, 2014, the Fund was named Oppenheimer Main Street Small- & Mid-Cap Fund.

Performance data quoted represents past performance, which does not guarantee future results. The investment return and principal value of an investment in the Fund will fluctuate so that an investor’s shares, when redeemed, may be worth more or less than their original cost. Fund returns include changes in share price, reinvested distributions, and a 5.75% maximum applicable sales charge except where “without sales charge” is indicated. Current performance may be lower or higher than the performance quoted. Returns do not consider capital gains or income taxes on an individual’s investment. Returns for periods of less than one year are cumulative and not annualized. For performance data current to the most recent month-end, visit oppenheimerfunds.com or call 1.800.CALL OPP (225.5677).

2       OPPENHEIMER MAIN STREET MID CAP FUND

Fund Performance Discussion

The Fund’s Class A shares (without sales charge) produced a cumulative total return of 3.31% during the reporting period. On a relative basis, the Russell Midcap Index (the “Index”) and the Russell 2500 Index produced returns of 4.18% and 1.06%, respectively. Relative to the Index, the Fund’s underperformance stemmed primarily from less favorable stock selection in the materials, consumer staples and industrials sectors. The Fund outperformed the Index in the information technology sector due to stronger relative stock selection and in the health care sector as a result of an overweight position.

MARKET OVERVIEW

During the past year domestic equity — as an asset class — once again distinguished itself as stocks appreciated in value across all capitalizations. In 2014, the U.S. economy provided a favorable backdrop for the outperformance of equities with steady, albeit modest growth, continued little wage inflation, and interest rates that remained low — in fact, well below initial expectations. Under these economic conditions most companies were able to demonstrate ongoing improvement with the majority beating expectations — on both the top and bottom lines.

The ride throughout the year, however, was anything but smooth. Macro influences — particularly the strengthening dollar, flattening yield curve, and tumbling oil price — played a significant role in determining which equities out- or underperformed. Accommodation by the Federal Reserve (the “Fed”), which officially ended its bond-buying stimulus program in October, helped to fuel demand for high-dividend paying stocks such as utilities and real estate investment trusts

(“REITs”), while a step-up in merger and acquisition activity — spurred on by the desire to relocate to advantageous low tax jurisdictions — boosted stocks primarily in the health care sector. But not all was rosy. The Fed’s accommodative behavior proved a headwind for many financials as long yields declined, hurting the profitability of banks. The strengthening U.S. dollar also proved a headwind, especially for multi-national companies where translation from local currencies to the greenback negatively impacted the bottom line. Consequently, stocks with international businesses broadly underperformed stocks with mostly domestic exposure.

Geopolitical risks, including those concentrated in Ukraine and Russia and the ever-present turmoil in the Middle East, caused investors to fret and resulted in, at times, quite the roller coaster ride. Add to this the fact that growth outside of the U.S. was anemic, at best, leading investors to worry intermittently about the global outlook and its impact on domestic stocks. And, a

3       OPPENHEIMER MAIN STREET MID CAP FUND

new concern raised its very ugly head as the Ebola epidemic in Western Africa spooked investors further. Though volatility remained below historical averages, these issues caused it to spike on a number of occasions throughout the year.

FUND REVIEW

Top contributors to performance this reporting period included Skyworks Solutions, Inc., Fidelity National Financial, Inc. and Ross Stores, Inc. Skyworks Solutions is a supplier of analog semiconductors to the wireless industry. The company’s stock benefited from the continued demand for higher bandwidth mobile devices, including Apple’s iPhone 6. High performance requirements have allowed Skyworks to leverage its broad portfolio of products to deliver more complex, higher value, integrated solutions versus the commodity, standalone parts of many of its competitors.

Fidelity National Financial, Inc. (FNF Group), the title insurance underwriter, benefited from growth of mortgage originations and refinancings as interest rates remained favorable. Additionally, management announced its intention to spin-off its Black Knight Financial Services subsidiary. The proceeds are expected to be used to pay down debt and augment growth — both of which should lead to rising earnings estimates. Ross Stores is a leading off-price apparel retailer that benefited as off-price retailers outperformed full-price retailers overall in 2014. The drop in gas prices also boosted Ross Stores and retailers in general,

due to the expected increase in consumer spending as a result.

Detractors from performance this reporting period included Noble Energy, Inc., Genworth Financial, Inc. and NOW, Inc. Noble has a high sensitivity to oil price changes as it has significant exploration and production operations globally. Growth projections are expected to decline as the company adjusts both its cash flow expectations and balance sheet to reflect a diminished outlook for global energy prices. We have maintained our position and continue to have confidence in management’s ability to execute well as of the end of the reporting period. Genworth, which provides insurance for the life, mortgage, and long-term care markets, disappointed investors when it reported third quarter results. Management announced a larger than projected charge to reflect a step-up in claims from holders of its long-term care policies. Additionally, the company disclosed that it would conduct a comprehensive review of all policies — spooking investors further. We think the company is currently undervalued by the market — especially given the sell-off in the stock — and we have maintained our position. NOW is a distributor of goods and services to the upstream, midstream and downstream energy and industrials industries. The difficult environment for the energy sector had a negative impact on NOW’s shares during the reporting period. We maintained our position.

4       OPPENHEIMER MAIN STREET MID CAP FUND

STRATEGY & OUTLOOK

While we have no crystal ball, we believe domestic equities should continue to be an attractive investment despite the above cross-currents. The U.S. economic outlook remains favorable, with ongoing moderate real growth, low inflationary pressures and little expected impact from the Fed’s initial tightening of the yield curve’s short-end. We believe profit margins, which have expanded to peak historical levels, are unlikely to expand significantly further, but quite probably can be sustained at current levels. With interest rates remaining low, combined with the fall in oil prices, consumer spending — which is the majority of GDP — should support our outlook for moderate growth.

Shocks to the market — whether economic or geopolitical in nature — will no doubt continue and could result in rising volatility. If true, we expect investors may favor stocks

characterized by “higher quality” — those that have pricing power, control over costs, and exhibit both improving returns on invested capital and generate excess cash flow. We believe this market environment can favor our consistent approach, which aims to construct an “all weather” portfolio by targeting companies we believe have: 1) sustainable competitive advantages; 2) skilled management with a proven track record of executing effectively; and 3) financial resources with the potential to generate improving profitability, gain market share, and/or return significant cash to shareholders. During times of volatile or slow economic growth such companies frequently widen their lead over weaker competitors. We seek to invest in companies characterized by these qualities at compelling valuations, and believe this disciplined approach is essential in seeking to generate superior long-term performance.

       Raymond Anello, CFA Portfolio Manager

Raymond Anello, CFA and Lead Portfolio Manager, on behalf of the Portfolio Management team: Matthew P. Ziehl, CFA, Raman Vardharaj, CFA, Joy Budzinski, Kristin Ketner, Magnus Krantz and Adam Weiner.

5       OPPENHEIMER MAIN STREET MID CAP FUND

Top Holdings and Allocations

TOP TEN COMMON STOCK HOLDINGS

Waste Connections, Inc.	3.7	%
Western Refining, Inc.	3.1
NOW, Inc.	2.9
Noble Energy, Inc.	2.2
Actavis plc	2.2
Universal Health Services, Inc., Cl. B	2.0
Dana Holding Corp.	2.0
Regions Financial Corp.	2.0
Applied Materials, Inc.	2.0
FNF Group	2.0

Portfolio holdings and allocations are subject to change. Percentages are as of December 31, 2014, and are based on net assets. For more current Fund holdings, please visit oppenheimerfunds.com.

TOP TEN COMMON STOCK INDUSTRIES

Oil, Gas & Consumable Fuels	7.1	%
Health Care Providers & Services	6.7
Commercial Banks	6.5
Real Estate Investment Trusts (REITs)	6.5
Commercial Services & Supplies	5.6
Software	5.3
Specialty Retail	5.2
Semiconductors & Semiconductor Equipment	4.2
Insurance	4.2
Pharmaceuticals	3.7

Portfolio holdings and allocations are subject to change. Percentages are as of December 31, 2014, and are based on net assets.

SECTOR ALLOCATION

Portfolio holdings and allocations are subject to change. Percentages are as of December 31, 2014, and are based on the total market value of common stocks.

6       OPPENHEIMER MAIN STREET MID CAP FUND

Share Class Performance

AVERAGE ANNUAL TOTAL RETURNS WITHOUT SALES CHARGE AS OF 12/31/14

	Inception Date	6-Month	1-Year	5-Year	10-Year
Class A (OPMSX)	8/2/99	3.31%	12.27%	15.96%	8.20%
Class B (OPMBX)	8/2/99	2.93%	11.44%	15.03%	7.71%
Class C (OPMCX)	8/2/99	2.91%	11.44%	15.09%	7.41%
Class I (OPMIX)	10/26/12	3.56%	12.78%	23.48% *	N/A
Class R (OPMNX)	3/1/01	3.19%	12.03%	15.66%	7.90%
Class Y (OPMYX)	8/2/99	3.45%	12.56%	16.38%	8.63%

AVERAGE ANNUAL TOTAL RETURNS WITH SALES CHARGE AS OF 12/31/14

	Inception Date	6-Month	1-Year	5-Year	10-Year
Class A (OPMSX)	8/2/99	-2.64%	5.82%	14.59%	7.57%
Class B (OPMBX)	8/2/99	-1.42%	6.73%	14.80%	7.71%
Class C (OPMCX)	8/2/99	2.04%	10.50%	15.09%	7.41%
Class I (OPMIX)	10/26/12	3.56%	12.78%	23.48% *	N/A
Class R (OPMNX)	3/1/01	2.31%	11.07%	15.66%	7.90%
Class Y (OPMYX)	8/2/99	3.45%	12.56%	16.38%	8.63%

*Shows performance since inception.

Performance data quoted represents past performance, which does not guarantee future results. The investment return and principal value of an investment in the Fund will fluctuate so that an investor’s shares, when redeemed, may be worth more or less than their original cost. Current performance may be lower or higher than the performance quoted. Returns do not consider capital gains or income taxes on an individual’s investment. For performance data current to the most recent month-end, visit oppenheimerfunds.com or call 1.800.CALL OPP (225.5677). Fund returns include changes in share price, reinvested distributions, and the applicable sales charge: for Class A shares, the current maximum initial sales charge of 5.75%; for Class B shares, the contingent deferred sales charge of 5% (1-year) and 2% (5-year); and for Class C shares, the contingent deferred sales charge (“CDSC”) of 1% for the 1-year period. Prior to 7/1/14, Class R shares were named Class N shares. Beginning 7/1/14, new purchases of Class R shares will no longer be subject to a CDSC upon redemption (any CDSC will remain in effect for purchases prior to 7/1/14). There is no sales charge for Class I and Class Y shares. Because Class B shares convert to Class A shares 72 months after purchase, the 10-year return for Class B shares uses Class A performance for the period after conversion. Returns for periods of less than one year are cumulative and not annualized.

The Fund’s performance is compared to the performance of the Russell Midcap Index and the Russell 2500 Index. The Russell Midcap Index measures the performance of the mid-cap segment of the U.S. equity universe. The Russell 2500 Index is a broad-based index featuring 2,500 stocks that cover the small- and mid-cap market capitalizations. The Fund has changed its benchmark from the Russell 2500 Index to the Russell Midcap Index, which it believes is a more appropriate measure of the Fund’s performance. The indices are unmanaged and cannot be purchased directly by investors. While index comparisons may be useful to provide a benchmark for the Fund’s performance, it must be noted that the Fund’s investments are

7       OPPENHEIMER MAIN STREET MID CAP FUND

not limited to the investments comprising the indices. Index performance includes reinvestment of income, but does not reflect transaction costs, fees, expenses or taxes. Index performance is shown for illustrative purposes only as a benchmark for the Fund’s performance, and does not predict or depict performance of the Fund. The Fund’s performance reflects the effects of the Fund’s business and operating expenses.

The Fund’s investment strategy and focus can change over time. The mention of specific fund holdings does not constitute a recommendation by OppenheimerFunds, Inc. or its affiliates.

Before investing in any of the Oppenheimer funds, investors should carefully consider a fund’s investment objectives, risks, charges and expenses. Fund prospectuses and summary prospectuses contain this and other information about the funds, and may be obtained by asking your financial advisor, visiting oppenheimerfunds.com, or calling 1.800.CALL OPP (225.5677). Read prospectuses and summary prospectuses carefully before investing.

Shares of Oppenheimer funds are not deposits or obligations of any bank, are not guaranteed by any bank, are not insured by the FDIC or any other agency, and involve investment risks, including the possible loss of the principal amount invested.

8       OPPENHEIMER MAIN STREET MID CAP FUND

Fund Expenses

Fund Expenses. As a shareholder of the Fund, you incur two types of costs: (1) transaction costs, which may include sales charges (loads) on purchase payments, contingent deferred sales charges on redemptions; and (2) ongoing costs, including management fees; distribution and service fees; and other Fund expenses. These examples are intended to help you understand your ongoing costs (in dollars) of investing in the Fund and to compare these costs with the ongoing costs of investing in other mutual funds.

The examples are based on an investment of $1,000.00 invested at the beginning of the period and held for the entire 6-month period ended December 31, 2014.

Actual Expenses. The first section of the table provides information about actual account values and actual expenses. You may use the information in this section for the class of shares you hold, together with the amount you invested, to estimate the expense that you paid over the period. Simply divide your account value by $1,000.00 (for example, an $8,600.00 account value divided by $1,000.00 = 8.60), then multiply the result by the number in the first section under the heading entitled “Expenses Paid During 6 Months Ended December 31, 2014” to estimate the expenses you paid on your account during this period.

Hypothetical Example for Comparison Purposes. The second section of the table provides information about hypothetical account values and hypothetical expenses based on the Fund’s actual expense ratio for each class of shares, and an assumed rate of return of 5% per year for each class before expenses, which is not the Fund’s actual return. The hypothetical account values and expenses may not be used to estimate the actual ending account balance or expenses you paid for the period. You may use this information to compare the ongoing costs of investing in the Fund and other funds. To do so, compare this 5% hypothetical example for the class of shares you hold with the 5% hypothetical examples that appear in the shareholder reports of the other funds.

Please note that the expenses shown in the table are meant to highlight your ongoing costs only and do not reflect any transactional costs, such as front-end or contingent deferred sales charges (loads). Therefore, the “hypothetical” section of the table is useful in comparing ongoing costs only, and will not help you determine the relative total costs of owning different funds. In addition, if these transactional costs were included, your costs would have been higher.

9       OPPENHEIMER MAIN STREET MID CAP FUND

Actual	Beginning Account Value July 1, 2014	Ending Account Value December 31, 2014	Expenses Paid During 6 Months Ended December 31, 2014
Class A	$    1,000.00	$    1,033.10	$          5.65
Class B	1,000.00	1,029.30	9.56
Class C	1,000.00	1,029.10	9.51
Class I	1,000.00	1,035.60	3.39
Class R	1,000.00	1,031.90	6.94
Class Y	1,000.00	1,034.50	4.37
Hypothetical (5% return before expenses)
Class A	1,000.00	1,019.66	5.62
Class B	1,000.00	1,015.83	9.49
Class C	1,000.00	1,015.88	9.44
Class I	1,000.00	1,021.88	3.37
Class R	1,000.00	1,018.40	6.89
Class Y	1,000.00	1,020.92	4.34

Expenses are equal to the Fund’s annualized expense ratio for that class, multiplied by the average account value over the period, multiplied by 184/365 (to reflect the one-half year period). Those annualized expense ratios, excluding indirect expenses from affiliated funds, based on the 6-month period ended December 31, 2014 are as follows:

Class	Expense Ratios
Class A		1.10	%
Class B		1.86
Class C		1.85
Class I		0.66
Class R		1.35
Class Y		0.85

The expense ratios reflect voluntary and/or contractual waivers and/or reimbursements of expenses by the Fund’s Manager and Transfer Agent. Some of these undertakings may be modified or terminated at any time, as indicated in the Fund’s prospectus. The “Financial Highlights” tables in the Fund’s financial statements, included in this report, also show the gross expense ratios, without such waivers or reimbursements and reduction to custodian expenses, if applicable.

10       OPPENHEIMER MAIN STREET MID CAP FUND

STATEMENT OF INVESTMENTS December 31, 2014 Unaudited

	Shares		Value
Common Stocks—97.0%
Consumer Discretionary—15.3%
Auto Components—2.5%
Dana Holding Corp.		3,424,940	$	74,458,195
Delphi Automotive plc		246,140		17,899,301
				92,357,496
Hotels, Restaurants & Leisure—2.2%
Brinker International, Inc.		365,150		21,430,653
Dunkin’ Brands Group, Inc.		1,380,535		58,879,818
				80,310,471
Household Durables—1.4%
Toll Brothers, Inc.1		1,524,159		52,232,929
Media—1.5%
IMAX Corp.1		1,775,510		54,863,259
Specialty Retail—5.2%
O’Reilly Automotive, Inc.1		258,090		49,713,296
Ross Stores, Inc.		550,870		51,925,006
Sally Beauty Holdings, Inc.1		1,510,050		46,418,937
Signet Jewelers Ltd.		299,480		39,402,584
				187,459,823
Textiles, Apparel & Luxury Goods—2.5%
Fossil Group, Inc.1		346,381		38,358,232
PVH Corp.		400,701		51,357,847
				89,716,079
Consumer Staples—2.5%
Food & Staples Retailing—0.8%
Rite Aid Corp.1		3,723,671		28,002,006
Food Products—1.7%
Flowers Foods, Inc.		3,185,051		61,121,129
Energy—8.8%
Energy Equipment & Services—1.7%
National Oilwell Varco, Inc.		960,890		62,967,122

	Shares		Value
Oil, Gas & Consumable Fuels—7.1%
Antero Midstream Partners LP1		1,130,171	$	31,079,702
Noble Energy, Inc.		1,717,050		81,439,682
Shell Midstream Partners LP1		719,830		29,498,633
Western Refining, Inc.		3,032,440		114,565,583
				256,583,600
Financials—19.9%
Commercial Banks—6.5%
CIT Group, Inc.		754,240		36,075,299
Comerica, Inc.		1,468,200		68,770,488
Commerce Bancshares, Inc.		109,281		4,752,631
Huntington Bancshares, Inc.		5,181,943		54,514,040
Regions Financial Corp.		6,884,940		72,704,967
				236,817,425
Consumer Finance—1.0%
Discover Financial Services		582,650		38,157,748
Diversified Financial Services—1.7%
CME Group, Inc., Cl. A		678,874		60,182,180
Insurance—4.2%
FNF Group		2,081,830		71,719,043
Genworth Financial, Inc., Cl. A1		4,679,450		39,775,325
Lincoln National Corp.		721,185		41,590,739
				153,085,107
Real Estate Investment Trusts (REITs)—6.5%
Digital Realty Trust, Inc.		572,613		37,964,242
Lamar Advertising Co., Cl. A		607,660		32,594,883
Mid-America Apartment Communities, Inc.		708,819		52,934,603
Outfront Media, Inc.		1,656,192		44,452,193

11       OPPENHEIMER MAIN STREET MID CAP FUND

STATEMENT OF INVESTMENTS Unaudited / Continued

	Shares		Value
Real Estate Investment Trusts (REITs) (Continued)
Starwood Property Trust, Inc.		2,957,876	$	68,741,038
				236,686,959
Health Care—13.8%
Biotechnology—1.3%
BioMarin Pharmaceutical, Inc.1		300,130		27,131,752
Celldex Therapeutics, Inc.1		529,720		9,667,390
Puma Biotechnology, Inc.1		51,000		9,652,770
				46,451,912
Health Care Equipment & Supplies—0.5%
DexCom, Inc.1		366,801		20,192,395
Health Care Providers & Services—6.7%
Centene Corp.1		285,592		29,658,729
Envision Healthcare Holdings, Inc.1		1,770,410		61,415,523
HealthSouth Corp.		1,065,504		40,979,284
Omnicare, Inc.		507,393		37,004,172
Universal Health Services, Inc., Cl. B		667,519		74,268,164
				243,325,872
Health Care Technology—1.2%
HMS Holdings Corp.1		2,053,552		43,412,089
Life Sciences Tools & Services—0.4%
Illumina, Inc.1		75,234		13,886,692
Pharmaceuticals—3.7%
Actavis plc1		310,640		79,961,842
Salix Pharmaceuticals Ltd.1		495,103		56,907,139
				136,868,981

	Shares		Value
Industrials—14.9%
Aerospace & Defense—1.2%
L-3 Communications Holdings, Inc.		342,280	$	43,199,159
Commercial Services & Supplies—5.6%
Tyco International plc		1,559,287		68,390,328
Waste Connections, Inc.		3,100,678		136,398,825
				204,789,153
Electrical Equipment—0.5%
Generac Holdings, Inc.1		395,840		18,509,478
Machinery—1.0%
Joy Global, Inc.		771,329		35,882,225
Professional Services—2.2%
On Assignment, Inc.1		841,110		27,916,441
Robert Half International, Inc.		922,520		53,856,718
				81,773,159
Road & Rail—1.5%
Genesee & Wyoming, Inc., Cl. A1		591,830		53,217,354
Trading Companies & Distributors—2.9%
NOW, Inc.1		4,100,806		105,513,738
Information Technology—13.2%
Internet Software & Services—0.8%
Pandora Media, Inc.1		1,644,890		29,328,389
IT Services—1.4%
Amdocs Ltd.		496,465		23,162,575
Xerox Corp.		2,015,820		27,939,265
				51,101,840
Semiconductors & Semiconductor Equipment—4.2%
Applied Materials, Inc.		2,897,950		72,216,914
Cavium, Inc.1		674,530		41,699,445
Skyworks Solutions, Inc.		554,014		40,282,358
				154,198,717
Software—5.3%
Fortinet, Inc.1		1,665,876		51,075,758

12       OPPENHEIMER MAIN STREET MID CAP FUND

	Shares		Value
Software (Continued)
Guidewire Software, Inc.1		1,229,235	$	62,236,168
PTC, Inc.1		731,064		26,793,496
ServiceNow, Inc.1		504,110		34,203,863
Splunk, Inc.1		278,460		16,415,217
				190,724,502
Technology Hardware, Storage & Peripherals—1.5%
Western Digital Corp.		492,182		54,484,547
Materials—7.0%
Chemicals—2.8%
Eastman Chemical Co.		783,010		59,399,138
International Flavors & Fragrances, Inc.		416,380		42,204,277
				101,603,415
Construction Materials—1.0%
Eagle Materials, Inc.		456,900		34,738,107
Containers & Packaging—0.9%
Packaging Corp. of America		412,192		32,171,586

	Shares		Value
Metals & Mining—2.3%
Alcoa, Inc.		4,344,070	$	68,592,865
United States Steel Corp.		624,520		16,699,665
				85,292,530
Utilities—1.6%
Electric Utilities—1.0%
Exelon Corp.		1,029,302		38,166,518
Water Utilities—0.6%
Aqua America, Inc.		769,472		20,544,902
Total Common Stocks (Cost $2,845,505,080)				3,529,920,593
Investment Company—3.7%
Oppenheimer Institutional Money Market Fund, Cl. E, 0.10%2,3 (Cost $136,374,357)		136,374,357		136,374,357
Total Investments, at Value (Cost $2,981,879,437)		100.7%		3,666,294,950
Net Other Assets (Liabilities)		(0.7)		(25,147,749	)
Net Assets		100.0%	$	3,641,147,201

Footnotes to Statement of Investments

1. Non-income producing security.

2. Is or was an affiliate, as defined in the Investment Company Act of 1940, as amended, at or during the period ended December 31, 2014, by virtue of the Fund owning at least 5% of the voting securities of the issuer or as a result of the Fund and the issuer having the same investment adviser. Transactions during the period in which the issuer was an affiliate are as follows:

	Shares June 30, 2014		Gross Additions		Gross Reductions		Shares December 31, 2014
Oppenheimer Institutional Money Market Fund, Cl. E		135,970,960		844,671,170		844,267,773		136,374,357
					Value		Income
Oppenheimer Institutional Money Market Fund, Cl. E					$	136,374,357	$	50,716

3. Rate shown is the 7-day yield as of December 31, 2014.

See accompanying Notes to Financial Statements.

13       OPPENHEIMER MAIN STREET MID CAP FUND

STATEMENT OF ASSETS AND LIABILITIES December 31, 2014 Unaudited

Assets
Investments, at value—see accompanying statement of investments:
Unaffiliated companies (cost $2,845,505,080)	$	3,529,920,593
Affiliated companies (cost $136,374,357)		136,374,357
		3,666,294,950
Cash		710,660
Receivables and other assets:
Investments sold		18,400,433
Dividends		5,350,457
Shares of beneficial interest sold		2,191,349
Other		150,196
Total assets		3,693,098,045
Liabilities
Payables and other liabilities:
Investments purchased		34,136,439
Shares of beneficial interest redeemed		17,072,701
Distribution and service plan fees		533,063
Trustees’ compensation		146,683
Shareholder communications		22,741
Other		39,217
Total liabilities		51,950,844
Net Assets	$	3,641,147,201
Composition of Net Assets
Par value of shares of beneficial interest	$	123,739
Additional paid-in capital		2,806,373,620
Accumulated net investment income		14,737,265
Accumulated net realized gain on investments		135,497,064
Net unrealized appreciation on investments		684,415,513
Net Assets	$	3,641,147,201

14       OPPENHEIMER MAIN STREET MID CAP FUND

Net Asset Value Per Share
Class A Shares:
Net asset value and redemption price per share (based on net assets of $1,865,294,801 and 63,221,387 shares of beneficial interest outstanding)	$	29.50
Maximum offering price per share (net asset value plus sales charge of 5.75% of offering price)	$	31.30
Class B Shares:
Net asset value, redemption price (excludes applicable contingent deferred sales charge) and offering price per share (based on net assets of $42,388,123 and 1,654,023 shares of beneficial interest outstanding)	$	25.63
Class C Shares:
Net asset value, redemption price (excludes applicable contingent deferred sales charge) and offering price per share (based on net assets of $391,394,240 and 15,156,927 shares of beneficial interest outstanding)	$	25.82
Class I Shares:
Net asset value, redemption price and offering price per share (based on net assets of $453,725,732 and 14,546,343 shares of beneficial interest outstanding)	$	31.19
Class R Shares:
Net asset value, redemption price (excludes applicable contingent deferred sales charge) and offering price per share (based on net assets of $229,411,592 and 8,084,360 shares of beneficial interest outstanding)	$	28.38
Class Y Shares:
Net asset value, redemption price and offering price per share (based on net assets of $658,932,713 and 21,076,139 shares of beneficial interest outstanding)	$	31.26

See accompanying Notes to Financial Statement.

15       OPPENHEIMER MAIN STREET MID CAP FUND

STATEMENT OF

OPERATIONS For the Six Months Ended December 31, 2014 Unaudited

Investment Income
Dividends:
Unaffiliated companies	$	36,646,142
Affiliated companies		50,716
Interest		315
Total investment income		36,697,173
Expenses
Management fees		11,731,165
Distribution and service plan fees:
Class A		2,328,301
Class B		228,021
Class C		1,951,100
Class R1		572,622
Transfer and shareholder servicing agent fees:
Class A		2,080,138
Class B		50,344
Class C		430,107
Class I		86,130
Class R1		252,405
Class Y		701,540
Shareholder communications:
Class A		28,609
Class B		2,681
Class C		6,253
Class I		11
Class R1		1,349
Class Y		996
Trustees’ compensation		55,215
Custodian fees and expenses		9,465
Other		79,933
Total expenses		20,596,385
Less reduction to custodian expenses		(764)
Less waivers and reimbursements of expenses		(55,817)
Net expenses		20,539,804
Net Investment Income		16,157,369
Realized and Unrealized Gain (Loss)
Net realized gain on investments from unaffiliated companies		276,267,646
Net change in unrealized appreciation/depreciation on investments		(174,549,249)
Net Increase in Net Assets Resulting from Operations	$	117,875,766

1. Effective July 1, 2014, Class N shares were renamed Class R. See Note 1 of the accompanying Notes.

See accompanying Notes to Financial Statements.

16       OPPENHEIMER MAIN STREET MID CAP FUND

STATEMENTS OF CHANGES IN NET ASSETS

	Six Months Ended December 31, 2014 (Unaudited)		Year Ended June 30, 2014
Operations
Net investment income	$	16,157,369	$	21,375,999
Net realized gain		276,267,646		672,136,333
Net change in unrealized appreciation/depreciation		(174,549,249)		201,805,806
Net increase in net assets resulting from operations		117,875,766		895,318,138
Dividends and/or Distributions to Shareholders
Dividends from net investment income:
Class A		(10,359,305)		(892,711)
Class B		—		—
Class C		—		—
Class I		(4,253,234)		(2,812,218)
Class R1		(804,704)		—
Class Y		(5,396,057)		(1,523,584)
		(20,813,300)		(5,228,513)
Distributions from net realized gain:
Class A		(250,861,303)		—
Class B		(6,415,183)		—
Class C		(59,110,058)		—
Class I		(57,437,945)		—
Class R1		(31,786,566)		—
Class Y		(87,269,685)		—
		(492,880,740)		—
Beneficial Interest Transactions
Net increase (decrease) in net assets resulting from beneficial interest transactions:
Class A		67,116,916		(164,574,172)
Class B		(3,837,199)		(19,537,242)
Class C		42,272,623		(23,429,544)
Class I		(135,195,215)		154,741,445
Class R1		18,912,858		(44,034,688)
Class Y		126,497,456		(357,296,104)
		115,767,439		(454,130,305)
Net Assets
Total increase (decrease)		(280,050,835)		435,959,320
Beginning of period		3,921,198,036		3,485,238,716
End of period (including accumulated net investment income of $14,737,265 and $19,393,196, respectively)	$	3,641,147,201	$	3,921,198,036

1. Effective July 1, 2014, Class N shares were renamed Class R. See Note 1 of the accompanying Notes.

See accompanying Notes to Financial Statements.

17       OPPENHEIMER MAIN STREET MID CAP FUND

FINANCIAL HIGHLIGHTS

Class A	Six Months Ended December 31, 2014 (Unaudited)		Year Ended June 30, 2014		Year Ended June 28, 2013 1		Year Ended June 29, 2012 1		Year Ended June 30, 2011		Year Ended June 30, 2010
Per Share Operating Data
Net asset value, beginning of period	$	33.30	$	26.17	$	21.75	$	21.99	$	16.33	$	13.70
Income (loss) from investment operations:
Net investment income2		0.14		0.17		0.15		0.04		0.05		0.02
*Net realized and unrealized gain (loss)		0.82		6.97		4.45		(0.23)		5.61		2.64
Total from investment operations		0.96		7.14		4.60		(0.19)		5.66		2.66
Dividends and/or distributions to shareholders:
Dividends from net investment income		(0.19)		(0.01)		(0.18)		(0.05)		0.00		(0.03)
Distributions from net realized gain		(4.57)		0.00		0.00		0.00		0.00		0.00
Total dividends and/or distributions to shareholders		(4.76)		(0.01)		(0.18)		(0.05)		0.00		(0.03)
Net asset value, end of period	$	29.50	$	33.30	$	26.17	$	21.75	$	21.99	$	16.33
Total Return, at Net Asset Value3		3.31%		27.31%		21.30%		(0.85)%		34.66%		19.45%
Ratios/Supplemental Data
Net assets, end of period (in thousands)	$	1,865,295	$	1,999,887	$	1,716,475	$	1,662,531	$	2,026,656	$	1,849,907
Average net assets (in thousands)	$	1,875,779	$	1,876,128	$	1,708,977	$	1,696,301	$	2,016,616	$	1,988,573
Ratios to average net assets:4
Net investment income		0.87%		0.57%		0.62%		0.20%		0.28%		0.13%
Total expenses5		1.10%		1.12%		1.19%		1.24%		1.25%		1.31%
Expenses after payments, waivers and/or reimbursements and reduction to custodian expenses		1.10%		1.12%		1.19%		1.24%		1.24%		1.25%
Portfolio turnover rate		36%		63%		101%		81%		86%		101%

18       OPPENHEIMER MAIN STREET MID CAP FUND

1. June 28, 2013 and June 29, 2012 represent the last business day of the Fund’s reporting periods.

2. Per share amounts calculated based on the average shares outstanding during the period.

3. Assumes an initial investment on the business day before the first day of the fiscal period, with all dividends and distributions reinvested in additional shares on the reinvestment date, and redemption at the net asset value calculated on the last business day of the fiscal period. Sales charges are not reflected in the total returns. Total returns are not annualized for periods less than one full year. Returns do not reflect the deduction of taxes that a shareholder would pay on fund distributions or the redemption of fund shares.

4. Annualized for periods less than one full year.

5. Total expenses including indirect expenses from affiliated fund fees and expenses were as follows:

Six Months Ended December 31, 2014	1.10	%
Year Ended June 30, 2014	1.12	%
Year Ended June 28, 2013	1.19	%
Year Ended June 29, 2012	1.24	%
Year Ended June 30, 2011	1.25	%
Year Ended June 30, 2010	1.31	%

See accompanying Notes to Financial Statements.

19       OPPENHEIMER MAIN STREET MID CAP FUND

FINANCIAL HIGHLIGHTS Continued

Class B	Six Months Ended December 31, 2014 (Unaudited)		Year Ended June 30, 2014		Year Ended June 28, 2013 1		Year Ended June 29, 2012 1		Year Ended June 30, 2011		Year Ended June 30, 2010
Per Share Operating Data
Net asset value, beginning of period	$	29.47	$	23.33	$	19.40	$	19.72	$	14.77	$	12.46
Income (loss) from investment operations:
Net investment income (loss)2		0.01		(0.06)		(0.05)		(0.11)		(0.09)		(0.10)
Net realized and unrealized gain (loss)		0.72		6.20		3.98		(0.21)		5.04		2.41
Total from investment operations		0.73		6.14		3.93		(0.32)		4.95		2.31
Dividends and/or distributions to shareholders:
Dividends from net investment income		0.00		0.00		0.00		0.00		0.00		0.00
Distributions from net realized gain		(4.57)		0.00		0.00		0.00		0.00		0.00
Total dividends and/or distributions to shareholders		(4.57)		0.00		0.00		0.00		0.00		0.00
Net asset value, end of period	$	25.63	$	29.47	$	23.33	$	19.40	$	19.72	$	14.77
Total Return, at Net Asset Value3		2.93%		26.32%		20.26%		(1.62)%		33.51%		18.54%
Ratios/Supplemental Data
Net assets, end of period (in thousands)	$	42,388	$	51,449	$	57,999	$	69,088	$	102,861	$	106,797
Average net assets (in thousands)	$	45,366	$	55,373	$	62,574	$	76,237	$	107,948	$	129,492
Ratios to average net assets:4
Net investment income (loss)		0.07%		(0.24)%		(0.22)%		(0.63)%		(0.51)%		(0.65)%
Total expenses5		1.86%		1.98%		2.26%		2.33%		2.29%		2.35%
Expenses after payments, waivers and/or reimbursements and reduction to custodian expenses		1.86%		1.91%		2.06%		2.06%		2.04%		2.05%
Portfolio turnover rate		36%		63%		101%		81%		86%		101%

1. June 28, 2013 and June 29, 2012 represent the last business day of the Fund’s reporting periods.

2. Per share amounts calculated based on the average shares outstanding during the period.

3. Assumes an initial investment on the business day before the first day of the fiscal period, with all dividends and distributions reinvested in additional shares on the reinvestment date, and redemption at the net asset value calculated on the last business day of the fiscal period. Sales charges are not reflected in the total returns. Total returns are not annualized for periods less than one full year. Returns do not reflect the deduction of taxes that a shareholder would pay on fund distributions or the redemption of fund shares.

4. Annualized for periods less than one full year.

5. Total expenses including indirect expenses from affiliated fund fees and expenses were as follows:

Six Months Ended December 31, 2014	1.86%
Year Ended June 30, 2014	1.98%
Year Ended June 28, 2013	2.26%
Year Ended June 29, 2012	2.33%
Year Ended June 30, 2011	2.29%
Year Ended June 30, 2010	2.35%

See accompanying Notes to Financial Statements.

20       OPPENHEIMER MAIN STREET MID CAP FUND

Class C	Six Months Ended December 31, 2014 (Unaudited)		Year Ended June 30, 2014		Year Ended June 28, 2013 1		Year Ended June 29, 2012 1		Year Ended June 30, 2011		Year Ended June 30, 2010
Per Share Operating Data
Net asset value, beginning of period	$	29.66	$	23.47	$	19.53	$	19.84	$	14.85	$	12.53
Income (loss) from investment operations:
Net investment income (loss)2		0.02		(0.05)		(0.03)		(0.10)		(0.08)		(0.09)
Net realized and unrealized gain (loss)		0.71		6.24		4.01		(0.21)		5.07		2.41
Total from investment operations		0.73		6.19		3.98		(0.31)		4.99		2.32
Dividends and/or distributions to shareholders:
Dividends from net investment income		0.00		0.00		(0.04)		0.00		0.00		0.00
Distributions from net realized gain		(4.57)		0.00		0.00		0.00		0.00		0.00
Total dividends and/or distributions to shareholders		(4.57)		0.00		(0.04)		0.00		0.00		0.00
Net asset value, end of period	$	25.82	$	29.66	$	23.47	$	19.53	$	19.84	$	14.85
Total Return, at Net Asset Value3		2.91%		26.38%		20.39%		(1.56)%		33.60%		18.52%
Ratios/Supplemental Data
Net assets, end of period (in thousands)	$	391,394	$	397,240	$	334,870	$	320,566	$	384,059	$	329,180
Average net assets (in thousands)	$	387,959	$	370,920	$	326,360	$	327,119	$	369,499	$	350,612
Ratios to average net assets:4
Net investment income (loss)		0.13%		(0.18)%		(0.13)%		(0.56)%		(0.45)%		(0.63)%
Total expenses5		1.85%		1.86%		1.94%		2.00%		1.99%		2.06%
Expenses after payments, waivers and/or reimbursements and reduction to custodian expenses		1.85%		1.86%		1.94%		2.00%		1.97%		2.01%
Portfolio turnover rate		36%		63%		101%		81%		86%		101%

1. June 28, 2013 and June 29, 2012 represent the last business day of the Fund’s reporting periods.

2. Per share amounts calculated based on the average shares outstanding during the period.

3. Assumes an initial investment on the business day before the first day of the fiscal period, with all dividends and distributions reinvested in additional shares on the reinvestment date, and redemption at the net asset value calculated on the last business day of the fiscal period. Sales charges are not reflected in the total returns. Total returns are not annualized for periods less than one full year. Returns do not reflect the deduction of taxes that a shareholder would pay on fund distributions or the redemption of fund shares.

4. Annualized for periods less than one full year.

5. Total expenses including indirect expenses from affiliated fund fees and expenses were as follows:

Six Months Ended December 31, 2014	1.85%
Year Ended June 30, 2014	1.86%
Year Ended June 28, 2013	1.94%
Year Ended June 29, 2012	2.00%
Year Ended June 30, 2011	1.99%
Year Ended June 30, 2010	2.06%

See accompanying Notes to Financial Statements.

21       OPPENHEIMER MAIN STREET MID CAP FUND

FINANCIAL HIGHLIGHTS Continued

Class I	Six Months Ended December 31, 2014 (Unaudited)		Year Ended June 30, 2014		Period Ended June 28, 20131,2
Per Share Operating Data
Net asset value, beginning of period	$	35.00	$	27.52	$	23.31
Income (loss) from investment operations:
Net investment income3		0.20		0.35		0.20
Net realized and unrealized gain		0.90		7.29		4.33
Total from investment operations		1.10		7.64		4.53
Dividends and/or distributions to shareholders:
Dividends from net investment income		(0.34)		(0.16)		(0.32)
Distributions from net realized gain		(4.57)		0.00		0.00
Total dividends and/or distributions to shareholders		(4.91)		(0.16)		(0.32)
Net asset value, end of period	$	31.19	$	35.00	$	27.52
Total Return, at Net Asset Value4		3.56%		27.83%		19.65%
Ratios/Supplemental Data
Net assets, end of period (in thousands)	$	453,726	$	633,358	$	362,310
Average net assets (in thousands)	$	568,026	$	506,714	$	68,428
Ratios to average net assets:5
Net investment income		1.14%		1.11%		1.08%
Total expenses6		0.66%		0.66%		0.66%
Expenses after payments, waivers and/or reimbursements and reduction to custodian expenses		0.66%		0.66%		0.66%
Portfolio turnover rate		36%		63%		101%

1. June 28, 2013 represents the last business day of the Fund’s 2013 reporting period.

2. For the period from October 26, 2012 (inception of offering) to June 28, 2013.

3. Per share amounts calculated based on the average shares outstanding during the period.

4. Assumes an initial investment on the business day before the first day of the fiscal period, with all dividends and distributions reinvested in additional shares on the reinvestment date, and redemption at the net asset value calculated on the last business day of the fiscal period. Sales charges are not reflected in the total returns. Total returns are not annualized for periods less than one full year. Returns do not reflect the deduction of taxes that a shareholder would pay on fund distributions or the redemption of fund shares.

5. Annualized for periods less than one full year.

6. Total expenses including indirect expenses from affiliated fund fees and expenses were as follows:

Six Months Ended December 31, 2014	0.66%
Year Ended June 30, 2014	0.66%
Period Ended June 28, 2013	0.66%

See accompanying Notes to Financial Statements.

22       OPPENHEIMER MAIN STREET MID CAP FUND

Class R	Six Months Ended December 31, 2014 (Unaudited)		Year Ended June 30, 2014		Year Ended June 28, 2013 1		Year Ended June 29, 2012 1		Year Ended June 30, 2011		Year Ended June 30, 2010
Per Share Operating Data
Net asset value, beginning of period	$	32.18	$	25.35	$	21.07	$	21.30	$	15.86	$	13.32
Income (loss) from investment operations:
Net investment income (loss)2		0.10		0.08		0.08		(0.01)		0.01		(0.02)
Net realized and unrealized gain (loss)		0.79		6.75		4.31		(0.22)		5.43		2.57
Total from investment operations		0.89		6.83		4.39		(0.23)		5.44		2.55
Dividends and/or distributions to shareholders:
Dividends from net investment income		(0.12)		0.00		(0.11)		0.00		0.00		(0.01)
Distributions from net realized gain		(4.57)		0.00		0.00		0.00		0.00		0.00
Total dividends and/or distributions to shareholders		(4.69)		0.00		(0.11)		0.00		0.00		(0.01)
Net asset value, end of period	$	28.38	$	32.18	$	25.35	$	21.07	$	21.30	$	15.86
Total Return, at Net Asset Value3		3.19%		26.94%		20.94%		(1.08)%		34.30%		19.15%
Ratios/Supplemental Data
Net assets, end of period (in thousands)	$	229,411	$	236,139	$	224,448	$	227,261	$	273,682	$	242,128
Average net assets (in thousands)	$	227,657	$	234,597	$	221,263	$	232,102	$	269,808	$	244,540
Ratios to average net assets:4
Net investment income (loss)		0.63%		0.28%		0.33%		(0.07)%		0.03%		(0.11)%
Total expenses5		1.35%		1.39%		1.53%		1.57%		1.60%		1.68%
Expenses after payments, waivers and/or reimbursements and reduction to custodian expenses		1.35%		1.39%		1.49%		1.51%		1.49%		1.49%
Portfolio turnover rate		36%		63%		101%		81%		86%		101%

1. June 28, 2013 and June 29, 2012 represent the last business day of the Fund’s reporting periods.

2. Per share amounts calculated based on the average shares outstanding during the period.

3. Assumes an initial investment on the business day before the first day of the fiscal period, with all dividends and distributions reinvested in additional shares on the reinvestment date, and redemption at the net asset value calculated on the last business day of the fiscal period. Sales charges are not reflected in the total returns. Total returns are not annualized for periods less than one full year. Returns do not reflect the deduction of taxes that a shareholder would pay on fund distributions or the redemption of fund shares.

4. Annualized for periods less than one full year.

5. Total expenses including indirect expenses from affiliated fund fees and expenses were as follows:

Six Months Ended December 31, 2014	1.35%
Year Ended June 30, 2014	1.39%
Year Ended June 28, 2013	1.53%
Year Ended June 29, 2012	1.57%
Year Ended June 30, 2011	1.60%
Year Ended June 30, 2010	1.68%

See accompanying Notes to Financial Statements.

23       OPPENHEIMER MAIN STREET MID CAP FUND

FINANCIAL HIGHLIGHTS Continued

Class Y	Six Months Ended December 31, 2014 (Unaudited)		Year Ended June 30, 2014		Year Ended June 28, 2013 1		Year Ended June 29, 2012 1		Year Ended June 30, 2011		Year Ended June 30, 2010
Per Share Operating Data
Net asset value, beginning of period	$	35.05	$	27.53	$	22.88	$	23.12	$	17.17	$	14.38
Income (loss) from investment operations:
Net investment income2		0.20		0.24		0.25		0.13		0.14		0.10
Net realized and unrealized gain (loss)		0.86		7.36		4.68		(0.24)		5.89		2.77
Total from investment operations		1.06		7.60		4.93		(0.11)		6.03		2.87
Dividends and/or distributions to shareholders:
Dividends from net investment income		(0.28)		(0.08)		(0.28)		(0.13)		(0.08)		(0.08)
Distributions from net realized gain		(4.57)		0.00		0.00		0.00		0.00		0.00
Total dividends and/or distributions to shareholders		(4.85)		(0.08)		(0.28)		(0.13)		(0.08)		(0.08)
Net asset value, end of period	$	31.26	$	35.05	$	27.53	$	22.88	$	23.12	$	17.17
Total Return, at Net Asset Value3		3.45%		27.63%		21.74%		(0.40)%		35.14%		20.00%
Ratios/Supplemental Data
Net assets, end of period (in thousands)	$	658,933	$	603,125	$	789,137	$	908,339	$	1,059,058	$	828,624
Average net assets (in thousands)	$	633,026	$	686,659	$	977,581	$	904,802	$	992,951	$	914,353
Ratios to average net assets:4
Net investment income		1.17%		0.76%		1.00%		0.60%		0.69%		0.55%
Total expenses5		0.85%		0.84%		0.81%		0.85%		0.83%		0.83%
Expenses after payments, waivers and/or reimbursements and reduction to custodian expenses		0.85%		0.84%		0.81%		0.85%		0.83%		0.83%
Portfolio turnover rate		36%		63%		101%		81%		86%		101%

1. June 28, 2013 and June 29, 2012 represent the last business day of the Fund’s reporting periods.

2. Per share amounts calculated based on the average shares outstanding during the period.

3. Assumes an initial investment on the business day before the first day of the fiscal period, with all dividends and distributions reinvested in additional shares on the reinvestment date, and redemption at the net asset value calculated on the last business day of the fiscal period. Sales charges are not reflected in the total returns. Total returns are not annualized for periods less than one full year. Returns do not reflect the deduction of taxes that a shareholder would pay on fund distributions or the redemption of fund shares.

4. Annualized for periods less than one full year.

5. Total expenses including indirect expenses from affiliated fund fees and expenses were as follows:

Six Months Ended December 31, 2014	0.85	%
Year Ended June 30, 2014	0.84	%
Year Ended June 28, 2013	0.81	%
Year Ended June 29, 2012	0.85	%
Year Ended June 30, 2011	0.83	%
Year Ended June 30, 2010	0.83	%

See accompanying Notes to Financial Statements.

24       OPPENHEIMER MAIN STREET MID CAP FUND

NOTES TO FINANCIAL STATEMENTS December 31, 2014 Unaudited

1. Organization

Oppenheimer Main Street Mid Cap Fund (the “Fund”) is a diversified open-end management investment company registered under the Investment Company Act of 1940 (“1940 Act”), as amended. The Fund’s investment objective is to seek capital appreciation. The Fund’s investment adviser is OFI Global Asset Management, Inc. (“OFI Global” or the “Manager”), a wholly-owned subsidiary of OppenheimerFunds, Inc. (“OFI” or the “Sub-Adviser”). The Manager has entered into a sub-advisory agreement with OFI.

The Fund offers Class A, Class C, Class I, Class R and Class Y shares, and previously offered Class B shares for new purchase through June 29, 2012. Subsequent to that date, no new purchases of Class B shares are permitted, however reinvestment of dividend and/or capital gain distributions and exchanges of Class B shares into and from other Oppenheimer funds will be allowed. As of July 1, 2014, Class N shares were renamed Class R shares. Class N shares subject to a contingent deferred sales charge (“CDSC”) on July 1, 2014, will continue to be subject to a CDSC after the shares are renamed. Purchases of Class R shares occurring on or after July 1, 2014, will not be subject to a CDSC upon redemption. Class A shares are sold at their offering price, which is normally net asset value plus a front-end sales charge. Class C and Class R shares are sold, and Class B shares were sold, without a front-end sales charge but may be subject to a contingent deferred sales charge (“CDSC”). Class R shares are sold only through retirement plans. Retirement plans that offer Class R shares may impose charges on those accounts. Class I and Class Y shares are sold to certain institutional investors or intermediaries without either a front-end sales charge or a CDSC, however, the intermediaries may impose charges on their accountholders who beneficially own Class I and Class Y shares. All classes of shares have identical rights and voting privileges with respect to the Fund in general and exclusive voting rights on matters that affect that class alone. Earnings, net assets and net asset value per share may differ due to each class having its own expenses, such as transfer and shareholder servicing agent fees and shareholder communications, directly attributable to that class. Class A, B, C and R shares have separate distribution and/or service plans under which they pay fees. Class I and Class Y shares do not pay such fees. Class B shares will automatically convert to Class A shares 72 months after the date of purchase.

The following is a summary of significant accounting policies consistently followed by the Fund.

2. Significant Accounting Policies

Security Valuation. All investments in securities are recorded at their estimated fair value, as described in Note 3.

Allocation of Income, Expenses, Gains and Losses. Income, expenses (other than those attributable to a specific class), gains and losses are allocated on a daily basis to each class of shares based upon the relative proportion of net assets represented by such class. Operating expenses directly attributable to a specific class are charged against the operations of that class.

25       OPPENHEIMER MAIN STREET MID CAP FUND

NOTES TO FINANCIAL STATEMENTS Unaudited / Continued

2. Significant Accounting Policies (Continued)

Dividends and Distributions to Shareholders. Dividends and distributions to shareholders, which are determined in accordance with income tax regulations and may differ from U.S. generally accepted accounting principles, are recorded on the ex-dividend date. Income and capital gain distributions, if any, are declared and paid annually or at other times as deemed necessary by the Manager. The tax character of distributions is determined as of the Fund’s fiscal year end. Therefore, a portion of the Fund’s distributions made to shareholders prior to the Fund’s fiscal year end may ultimately be categorized as a tax return of capital.

Investment Income. Dividend income is recorded on the ex-dividend date or upon ex-dividend notification in the case of certain foreign dividends where the ex-dividend date may have passed. Non-cash dividends included in dividend income, if any, are recorded at the fair market value of the securities received. Interest income is recognized on an accrual basis. Discount and premium, which are included in interest income on the Statement of Operations, are amortized or accreted daily.

Custodian Fees. “Custodian fees and expenses” in the Statement of Operations may include interest expense incurred by the Fund on any cash overdrafts of its custodian account during the period. Such cash overdrafts may result from the effects of failed trades in portfolio securities and from cash outflows resulting from unanticipated shareholder redemption activity. The Fund pays interest to its custodian on such cash overdrafts, to the extent they are not offset by positive cash balances maintained by the Fund, at a rate equal to the Federal Funds Rate plus 0.50%. The “Reduction to custodian expenses” line item, if applicable, represents earnings on cash balances maintained by the Fund during the period. Such interest expense and other custodian fees may be paid with these earnings.

Security Transactions. Security transactions are recorded on the trade date. Realized gains and losses on securities sold are determined on the basis of identified cost.

Indemnifications. The Fund’s organizational documents provide current and former Trustees and officers with a limited indemnification against liabilities arising in connection with the performance of their duties to the Fund. In the normal course of business, the Fund may also enter into contracts that provide general indemnifications. The Fund’s maximum exposure under these arrangements is unknown as this would be dependent on future claims that may be made against the Fund. The risk of material loss from such claims is considered remote.

Federal Taxes. The Fund intends to comply with provisions of the Internal Revenue Code applicable to regulated investment companies and to distribute substantially all of its investment company taxable income to shareholders. Therefore, no federal income or excise tax provision is required. The Fund files income tax returns in U.S. federal and applicable state jurisdictions. The statute of limitations on the Fund’s tax return filings generally remain open for the three preceding fiscal reporting period ends.

26       OPPENHEIMER MAIN STREET MID CAP FUND

2. Significant Accounting Policies (Continued)

During the fiscal year ended June 30, 2014, the Fund utilized $262,355,498 of capital loss carryforward to offset capital gains realized in that fiscal year. As of December 31, 2014, it is estimated that there would be no capital loss carryforwards. The estimated capital loss carryforward represents the carryforward as of the end of the last fiscal year, increased or decreased by capital losses or gains realized in the first six months of the current fiscal year. During the six months ended December 31, 2014, it is estimated that the Fund will not utilize any capital loss carryforward to offset realized capital gains.

Net investment income (loss) and net realized gain (loss) may differ for financial statement and tax purposes. The character of dividends and distributions made during the fiscal year from net investment income or net realized gains may differ from their ultimate characterization for federal income tax purposes. Also, due to timing of dividends and distributions, the fiscal year in which amounts are distributed may differ from the fiscal year in which the income or net realized gain was recorded by the Fund.

The aggregate cost of securities and other investments and the composition of unrealized appreciation and depreciation of securities and other investments for federal income tax purposes as of December 31, 2014 are noted in the following table. The primary difference between book and tax appreciation or depreciation of securities and other investments, if applicable, is attributable to the tax deferral of losses or tax realization of financial statement unrealized gain or loss.

Federal tax cost of securities	$	2,987,257,165
Gross unrealized appreciation	$	804,970,953
Gross unrealized depreciation		(125,933,168)
Net unrealized appreciation	$	679,037,785

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 disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of increases and decreases in net assets from operations during the reporting period. Actual results could differ from those estimates.

3. Securities Valuation

The Fund calculates the net asset value of its shares as of the close of the New York Stock Exchange (the “Exchange”), normally 4:00 P.M. Eastern time, on each day the Exchange is open for trading.

The Fund’s Board has adopted procedures for the valuation of the Fund’s securities and has delegated the day-to-day responsibility for valuation determinations under those procedures to the Manager. The Manager has established a Valuation Committee which is responsible for determining a “fair valuation” for any security for which market quotations are not “readily available.” The Valuation Committee’s fair valuation determinations are subject to review, approval and ratification by the Fund’s Board at its next regularly scheduled meeting covering the calendar quarter in which the fair valuation was determined.

27       OPPENHEIMER MAIN STREET MID CAP FUND

NOTES TO FINANCIAL STATEMENTS Unaudited / Continued

3. Securities Valuation (Continued)

Valuation Methods and Inputs

Securities are valued using unadjusted quoted market prices, when available, as supplied primarily by third party pricing services or dealers.

The following methodologies are used to determine the market value or the fair value of the types of securities described below:

Securities traded on a registered U.S. securities exchange (including exchange-traded derivatives other than futures and futures options) are valued based on the last sale price of the security reported on the principal exchange on which it is traded, prior to the time when the Fund’s assets are valued. In the absence of a sale, the security is valued at the last sale price on the prior trading day, if it is within the spread of the current day’s closing “bid” and “asked” prices, and if not, at the current day’s closing bid price. A security of a foreign issuer traded on a foreign exchange, but not listed on a registered U.S. securities exchange, is valued based on the last sale price on the principal exchange on which the security is traded, as identified by the third party pricing service used by the Manager, prior to the time when the Fund’s assets are valued. If the last sale price is unavailable, the security is valued at the most recent official closing price on the principal exchange on which it is traded. If the last sales price or official closing price for a foreign security is not available, the security is valued at the mean between the bid and asked price per the exchange or, if not available from the exchange, obtained from two dealers. If bid and asked prices are not available from either the exchange or two dealers, the security is valued by using one of the following methodologies (listed in order of priority): (1) using a bid from the exchange, (2) the mean between the bid and asked price as provided by a single dealer, or (3) a bid from a single dealer.

Shares of a registered investment company that are not traded on an exchange are valued at that investment company’s net asset value per share.

Corporate and government debt securities (of U.S. or foreign issuers) and municipal debt securities, event-linked bonds, loans, mortgage-backed securities, collateralized mortgage obligations, and asset-backed securities are valued at the mean between the “bid” and “asked” prices utilizing evaluated prices obtained from third party pricing services or broker-dealers who may use matrix pricing methods to determine the evaluated prices.

Short-term money market type debt securities with a remaining maturity of sixty days or less are valued at cost adjusted by the amortization of discount or premium to maturity (amortized cost), which approximates market value. Short-term debt securities with a remaining maturity in excess of sixty days are valued at the mean between the “bid” and “asked” prices utilizing evaluated prices obtained from third party pricing services or broker-dealers.

A description of the standard inputs that may generally be considered by the third party pricing vendors in determining their evaluated prices is provided below.

28       OPPENHEIMER MAIN STREET MID CAP FUND

3. Securities Valuation (Continued)

Security Type	Standard inputs generally considered by third-party pricing vendors
Corporate debt, government debt, municipal, mortgage-backed and asset-backed securities	Reported trade data, broker-dealer price quotations, benchmark yields, issuer spreads on comparable securities, the credit quality, yield, maturity, and other appropriate factors.
Loans	Information obtained from market participants regarding reported trade data and broker-dealer price quotations.
Event-linked bonds	Information obtained from market participants regarding reported trade data and broker-dealer price quotations.

If a market value or price cannot be determined for a security using the methodologies described above, or if, in the “good faith” opinion of the Manager, the market value or price obtained does not constitute a “readily available market quotation,” or a significant event has occurred that would materially affect the value of the security the security is fair valued either (i) by a standardized fair valuation methodology applicable to the security type or the significant event as previously approved by the Valuation Committee and the Fund’s Board or (ii) as determined in good faith by the Manager’s Valuation Committee. The Valuation Committee considers all relevant facts that are reasonably available, through either public information or information available to the Manager, when determining the fair value of a security. Fair value determinations by the Manager are subject to review, approval and ratification by the Fund’s Board at its next regularly scheduled meeting covering the calendar quarter in which the fair valuation was determined. Those fair valuation standardized methodologies include, but are not limited to, valuing securities at the last sale price or initially at cost and subsequently adjusting the value based on: changes in company specific fundamentals, changes in an appropriate securities index, or changes in the value of similar securities which may be further adjusted for any discounts related to security-specific resale restrictions. When possible, such methodologies use observable market inputs such as unadjusted quoted prices of similar securities, observable interest rates, currency rates and yield curves. The methodologies used for valuing securities are not necessarily an indication of the risks associated with investing in those securities nor can it be assured that the Fund can obtain the fair value assigned to a security if it were to sell the security.

To assess the continuing appropriateness of security valuations, the Manager, or its third party service provider who is subject to oversight by the Manager, regularly compares prior day prices, prices on comparable securities, and sale prices to the current day prices and challenges those prices exceeding certain tolerance levels with the third party pricing service or broker source. For those securities valued by fair valuations, whether through a standardized fair valuation methodology or a fair valuation determination, the Valuation Committee reviews and affirms the reasonableness of the valuations based on such methodologies and fair valuation determinations on a regular basis after considering all relevant information that is reasonably available.

Classifications

Each investment asset or liability of the Fund is assigned a level at measurement date based on the significance and source of the inputs to its valuation. Various data inputs are used in determining the value of each of the Fund’s investments as of the reporting period end. These

29       OPPENHEIMER MAIN STREET MID CAP FUND

NOTES TO FINANCIAL STATEMENTS Unaudited / Continued

3. Securities Valuation (Continued)

data inputs are categorized in the following hierarchy under applicable financial accounting standards:

1) Level 1-unadjusted quoted prices in active markets for identical assets or liabilities (including securities actively traded on a securities exchange)

2) Level 2-inputs other than unadjusted quoted prices that are observable for the asset or liability (such as unadjusted quoted prices for similar assets and market corroborated inputs such as interest rates, prepayment speeds, credit risks, etc.)

3) Level 3-significant unobservable inputs (including the Manager’s own judgments about assumptions that market participants would use in pricing the asset or liability).

The inputs used for valuing securities are not necessarily an indication of the risks associated with investing in those securities.

The table below categorizes amounts that are included in the Fund’s Statement of Assets and Liabilities as of December 31, 2014 based on valuation input level:

	Level 1— Unadjusted         Quoted Prices		Level 2— Other Significant Observable Inputs		Level 3— Significant         Unobservable Inputs		Value
Assets Table
Investments, at Value:
Common Stocks
Consumer Discretionary	$	556,940,057	$	—	$	—	$	556,940,057
Consumer Staples		89,123,135		—		—		89,123,135
Energy		319,550,722		—		—		319,550,722
Financials		724,929,419		—		—		724,929,419
Health Care		504,137,941		—		—		504,137,941
Industrials		542,884,266		—		—		542,884,266
Information Technology		479,837,995		—		—		479,837,995
Materials		253,805,638		—		—		253,805,638
Utilities		58,711,420		—		—		58,711,420
Investment Company		136,374,357		—		—		136,374,357
Total Assets	$	3,666,294,950	$	—	$	—	$	3,666,294,950

Forward currency exchange contracts and futures contracts, if any, are reported at their unrealized appreciation/depreciation at measurement date, which represents the change in the contract’s value from trade date. All additional assets and liabilities included in the above table are reported at their market value at measurement date.

4. Investments and Risks

Investment in Oppenheimer Institutional Money Market Fund. The Fund is permitted to invest daily available cash balances in an affiliated money market fund. The Fund may invest the available cash in Class E shares of Oppenheimer Institutional Money Market Fund (“IMMF”) to seek current income while preserving liquidity or for defensive purposes. IMMF is a registered open-end management investment company, regulated as a money market fund under the 1940 Act. The Manager is the investment adviser of IMMF, and the Sub-Adviser provides investment and related advisory services to IMMF. When applicable, the Fund’s

30       OPPENHEIMER MAIN STREET MID CAP FUND

4. Investments and Risks (Continued)

investment in IMMF is included in the Statement of Investments. Shares of IMMF are valued at their net asset value per share. As a shareholder, the Fund is subject to its proportional share of IMMF’s Class E expenses, including its management fee. The Manager will waive fees and/or reimburse Fund expenses in an amount equal to the indirect management fees incurred through the Fund’s investment in IMMF.

Equity Security Risk. Stocks and other equity securities fluctuate in price. The value of the Fund’s portfolio may be affected by changes in the equity markets generally. Equity markets may experience significant short-term volatility and may fall sharply at times. Different markets may behave differently from each other and U.S. equity markets may move in the opposite direction from one or more foreign stock markets.

The prices of individual equity securities generally do not all move in the same direction at the same time and a variety of factors can affect the price of a particular company’s securities. These factors may include, but are not limited to, poor earnings reports, a loss of customers, litigation against the company, general unfavorable performance of the company’s sector or industry, or changes in government regulations affecting the company or its industry.

5. Shares of Beneficial Interest

The Fund has authorized an unlimited number of $0.001 par value shares of beneficial interest of each class. Transactions in shares of beneficial interest were as follows:

	Six Months Ended December 31, 2014				Year Ended June 30, 2014
	Shares		Amount		Shares		Amount
Class A
Sold		4,177,498	$	135,462,796		8,053,385	$	239,302,251
Dividends and/or distributions reinvested		8,630,376		247,346,578		28,823		846,538
Redeemed		(9,645,528)		(315,692,458)		(13,607,293)		(404,722,961)
Net increase (decrease)		3,162,346	$	67,116,916		(5,525,085)	$	(164,574,172)
Class B
Sold		37,934	$	1,085,480		83,089	$	2,179,662
Dividends and/or distributions reinvested		254,424		6,337,698		—		—
Redeemed		(384,265)		(11,260,377)		(822,822)		(21,716,904)
Net decrease		(91,907)	$	(3,837,199)		(739,733)	$	(19,537,242)
Class C
Sold		932,790	$	26,299,598		1,732,140	$	46,203,993
Dividends and/or distributions reinvested		2,183,456		54,804,753		—		—
Redeemed		(1,353,823)		(38,831,728)		(2,603,567)		(69,633,537)
Net increase (decrease)		1,762,423	$	42,272,623		(871,427)	$	(23,429,544)

31       OPPENHEIMER MAIN STREET MID CAP FUND

NOTES TO FINANCIAL STATEMENTS Unaudited / Continued

5. Shares of Beneficial Interest (Continued)

	Six Months Ended December 31, 2014				Year Ended June 30, 2014
	Shares		Amount		Shares		Amount
Class I
Sold		751,108	$	25,911,792		7,644,932	$	241,172,541
Dividends and/or distributions reinvested		2,036,498		61,685,537		91,280		2,810,508
Redeemed		(6,334,833)		(222,792,544)		(2,809,756)		(89,241,604)
Net increase (decrease)		(3,547,227)	$	(135,195,215)		4,926,456	$	154,741,445
Class R1
Sold		828,088	$	25,764,058		1,712,426	$	48,896,112
Dividends and/or distributions reinvested		1,106,195		30,497,786		—		—
Redeemed		(1,189,104)		(37,348,986)		(3,228,681)		(92,930,800)
Net increase (decrease)		745,179	$	18,912,858		(1,516,255)	$	(44,034,688)
Class Y
Sold		5,250,609	$	182,964,553		3,728,341	$	115,986,149
Dividends and/or distributions reinvested		2,926,375		88,874,008		46,662		1,440,456
Redeemed		(4,307,641)		(145,341,105)		(15,231,450)		(474,722,709)
Net increase (decrease)		3,869,343	$	126,497,456		(11,456,447)	$	(357,296,104)

1. Effective July 1, 2014, Class N shares were renamed Class R.

6. Purchases and Sales of Securities

The aggregate cost of purchases and proceeds from sales of securities, other than short-term obligations and investments in IMMF, for the six months ended December 31, 2014 were as follows:

	Purchases		Sales
Investment securities	$	1,316,707,234	$	1,665,378,756

7. Fees and Other Transactions with Affiliates

Management Fees. Under the investment advisory agreement, the Fund pays the Manager a management fee based on the daily net assets of the Fund at an annual rate as shown in the following table:

Fee Schedule
Up to $200 million	0.75%
Next $200 million	0.72
Next $200 million	0.69
Next $200 million	0.66
Next $4.2 billion	0.60
Over $5.0 billion	0.58

The Fund’s management fee for the fiscal six months ended December 31, 2014 was 0.62% of average annual net assets before any applicable waivers.

32       OPPENHEIMER MAIN STREET MID CAP FUND

7. Fees and Other Transactions with Affiliates (Continued)

Sub-Adviser Fees. The Manager has retained the Sub-Adviser to provide the day-to-day portfolio management of the Fund. Under the Sub-Advisory Agreement, the Manager pays the Sub-Adviser an annual fee in monthly installments, equal to a percentage of the investment management fee collected by the Manager from the Fund, which shall be calculated after any investment management fee waivers. The fee paid to the Sub-Adviser is paid by the Manager, not by the Fund.

Transfer Agent Fees. OFI Global (the “Transfer Agent”) serves as the transfer and shareholder servicing agent for the Fund. The Fund pays the Transfer Agent a fee based on annual net assets. Fees incurred and average net assets for each class with respect to these services are detailed in the Statement of Operations and Financial Highlights, respectively.

Sub-Transfer Agent Fees. The Transfer Agent has retained Shareholder Services, Inc., a wholly-owned subsidiary of OFI (the “Sub-Transfer Agent”), to provide the day-to-day transfer agent and shareholder servicing of the Fund. Under the Sub-Transfer Agency Agreement, the Transfer Agent pays the Sub-Transfer Agent an annual fee in monthly installments, equal to a percentage of the transfer agent fee collected by the Transfer Agent from the Fund, which shall be calculated after any applicable fee waivers. The fee paid to the Sub-Transfer Agent is paid by the Transfer Agent, not by the Fund.

Trustees’ Compensation. The Fund’s Board of Trustees (“Board”) has adopted a compensation deferral plan for Independent Trustees that enables Trustees to elect to defer receipt of all or a portion of the annual compensation they are entitled to receive from the Fund. For purposes of determining the amount owed to the Trustee under the plan, deferred amounts are treated as though equal dollar amounts had been invested in shares of the Fund or in other Oppenheimer funds selected by the Trustee. The Fund purchases shares of the funds selected for deferral by the Trustee in amounts equal to his or her deemed investment, resulting in a Fund asset equal to the deferred compensation liability. Such assets are included as a component of “Other” within the asset section of the Statement of Assets and Liabilities. Deferral of Trustees’ fees under the plan will not affect the net assets of the Fund and will not materially affect the Fund’s assets, liabilities or net investment income per share. Amounts will be deferred until distributed in accordance with the compensation deferral plan.

Distribution and Service Plan (12b-1) Fees. Under its General Distributor’s Agreement with the Fund, OppenheimerFunds Distributor, Inc. (the “Distributor”) acts as the Fund’s principal underwriter in the continuous public offering of the Fund’s classes of shares.

Service Plan for Class A Shares. The Fund has adopted a Service Plan (the “Plan”) for Class A shares pursuant to Rule 12b-1 under the 1940 Act. Under the Plan, the Fund reimburses the Distributor for a portion of its costs incurred for services provided to accounts that hold Class A shares. Reimbursement is made periodically at an annual rate of up to 0.25% of the daily net assets of Class A shares of the Fund. The Distributor currently uses all of those fees to pay dealers, brokers, banks and other financial institutions periodically for

33       OPPENHEIMER MAIN STREET MID CAP FUND

NOTES TO FINANCIAL STATEMENTS Unaudited / Continued

7. Fees and Other Transactions with Affiliates (Continued)

providing personal service and maintenance of accounts of their customers that hold Class A shares. Any unreimbursed expenses the Distributor incurs with respect to Class A shares in any fiscal year cannot be recovered in subsequent periods. Fees incurred by the Fund under the Plan are detailed in the Statement of Operations.

Distribution and Service Plans for Class B, Class C and Class R Shares. The Fund has adopted Distribution and Service Plans (the “Plans”) for Class B, Class C and Class R shares pursuant to Rule 12b-1 under the 1940 Act to compensate the Distributor for distributing those share classes, maintaining accounts and providing shareholder services. Under the Plans, the Fund pays the Distributor an annual asset-based sales charge of 0.75% on Class B and Class C shares daily net assets and 0.25% on Class R shares daily net assets. The Fund also pays a service fee under the Plans at an annual rate of 0.25% of daily net assets. The Plans continue in effect from year to year only if the Fund’s Board of Trustees vote annually to approve its continuance at an in person meeting called for that purpose. Fees incurred by the Fund under the Plans are detailed in the Statement of Operations.

Sales Charges. Front-end sales charges and CDSC do not represent expenses of the Fund. They are deducted from the proceeds of sales of Fund shares prior to investment or from redemption proceeds prior to remittance, as applicable. The sales charges retained by the Distributor from the sale of shares and the CDSC retained by the Distributor on the redemption of shares is shown in the following table for the period indicated.

Year Ended	Class A Front-End Sales Charges Retained by Distributor	Class A Contingent Deferred Sales Charges Retained by Distributor	Class B Contingent Deferred Sales Charges Retained by Distributor	Class C Contingent Deferred Sales Charges Retained by Distributor	Class N Contingent Deferred Sales Charges Retained by Distributor
December 31, 2014	$165,826	$249	$14,905	$6,553	$1,746

Waivers and Reimbursements of Expenses. The Manager will waive fees and/or reimburse Fund expenses in an amount equal to the indirect management fees incurred through the Fund’s investment in IMMF. During the six months ended December 31, 2014, the Manager waived fees and/or reimbursed the Fund $55,817 for IMMF management fees.

These undertakings may be modified or terminated as set forth according to the terms in the prospectus.

8. Pending Litigation

In 2009, seven class action lawsuits were filed in the U.S. District Court for the District of Colorado against OppenheimerFunds, Inc. (“OFI”), OppenheimerFunds Distributor, Inc., the Fund’s principal underwriter and distributor (the “Distributor”), and certain funds (but not including the Fund) advised by OFI Global Asset Management, Inc. and distributed by the Distributor (the “Defendant Funds”). The lawsuits also named as defendants certain officers and current and former trustees of the respective Defendant Funds. The lawsuits raised claims

34       OPPENHEIMER MAIN STREET MID CAP FUND

8. Pending Litigation (Continued)

under federal securities laws and alleged, among other things, that the disclosure documents of the respective Defendant Funds contained misrepresentations and omissions and that the respective Defendant Funds’ investment policies were not followed. The plaintiffs in these actions sought unspecified damages, equitable relief and awards of attorneys’ fees and litigation expenses. The Defendant Funds’ Boards of Trustees also engaged counsel to represent the Funds and the present and former Independent Trustees named in those suits. In March 2014, the parties in six of these lawsuits executed stipulations and agreements of settlement resolving those actions. In July 2014, the court entered an order and final judgment approving the settlements as fair, reasonable and adequate. The settlements do not resolve a seventh outstanding lawsuit relating to Oppenheimer Rochester California Municipal Fund (the “California Fund Suit”). OFI believes the California Fund Suit is without legal merit and is defending the suit vigorously. While it is premature to render any opinion as to the outcome in the California Fund Suit, or whether any costs that OFI may bear in defending the California Fund Suit might not be reimbursed by insurance, OFI believes the California Fund Suit should not impair the ability of OFI or the Distributor to perform their respective duties to the Fund, and that the outcome of the California Fund Suit should not have any material effect on the operations of any of the Oppenheimer funds.

35       OPPENHEIMER MAIN STREET MID CAP FUND

BOARD APPROVAL OF THE FUND’S INVESTMENT ADVISORY

AND SUB-ADVISORY AGREEMENTS Unaudited

The Fund has entered into an investment advisory agreement with OFI Global Asset Management, Inc. (“OFI Global” or the “Adviser”), a wholly-owned subsidiary of OppenheimerFunds, Inc. (“OFI” or the “Sub-Adviser”) (“OFI Global” and “OFI” together the “Managers”) and OFI Global has entered into a sub-advisory agreement with OFI whereby OFI provides investment sub-advisory services to the Fund (collectively, the “Agreements”). Each year, the Board of Trustees (the “Board”), including a majority of the independent Trustees, is required to determine whether to renew the Agreements. The Investment Company Act of 1940, as amended, requires that the Board request and evaluate, and that the Managers provide, such information as may be reasonably necessary to evaluate the terms of the Agreements. The Board employs an independent consultant to prepare a report that provides information, including comparative information that the Board requests for that purpose. In addition to in-person meetings focused on this evaluation, the Board receives information throughout the year regarding Fund services, fees, expenses and performance.

The Adviser, Sub-Adviser and the independent consultant provided information to the Board on the following factors: (i) the nature, quality and extent of the Managers’ services, (ii) the investment performance of the Fund and the Managers, (iii) the fees and expenses of the Fund, including comparative expense information, (iv) the profitability of the Managers and their affiliates, including an analysis of the cost of providing services, (v) whether economies of scale are realized as the Fund grows and whether fee levels reflect these economies of scale for Fund investors and (vi) other benefits to the Managers from their relationship with the Fund. The Board was aware that there are alternatives to retaining the Managers.

Outlined below is a summary of the principal information considered by the Board as well as the Board’s conclusions.

Nature, Quality and Extent of Services. The Board considered information about the nature, quality and extent of the services provided to the Fund and information regarding the Managers’ key personnel who provide such services. The Sub-Adviser’s duties include providing the Fund with the services of the portfolio managers and the Sub-Adviser’s investment team, who provide research, analysis and other advisory services in regard to the Fund’s investments; securities trading services; oversight of third-party service providers; monitoring compliance with applicable Fund policies and procedures and adherence to the Fund’s investment restrictions; and risk management. The Managers are responsible for providing certain administrative services to the Fund as well. Those services include providing and supervising all administrative and clerical personnel who are necessary in order to provide effective corporate administration for the Fund; compiling and maintaining records with respect to the Fund’s operations; preparing and filing reports required by the Securities and Exchange Commission; preparing periodic reports regarding the operations of the Fund for its shareholders; preparing proxy materials for shareholder meetings; and preparing the registration statements required by Federal and state securities laws for the sale of the Fund’s shares. The Managers also provide the Fund with office space, facilities and equipment.

The Board also considered the quality of the services provided and the quality of the Managers’ resources that are available to the Fund. The Board took account of the fact that the Sub-Adviser has had over fifty years of experience as an investment adviser and that its assets under management rank it among the top mutual fund managers in the United States. The Board evaluated the Sub-Adviser’s advisory, administrative, accounting, legal, compliance

36       OPPENHEIMER MAIN STREET MID CAP FUND

services and risk management, and information the Board has received regarding the experience and professional qualifications of the Managers’ key personnel and the size and functions of its staff. In its evaluation of the quality of the portfolio management services provided, the Board considered the experience of Raymond Anello, Matthew Ziehl, Raman Vardharaj, Joy Budzinski, Kristin Ketner, Magnus Krantz, and Adam Weiner, the portfolio managers for the Fund, and the Sub-Adviser’s investment team and analysts. The Board members also considered the totality of their experiences with the Managers as directors or trustees of the Fund and other funds advised by the Managers. The Board considered information regarding the quality of services provided by affiliates of the Managers, which its members have become knowledgeable about in connection with the renewal of the Fund’s service agreements. The Board concluded, in light of the Managers’ experience, reputation, personnel, operations and resources that the Fund benefits from the services provided under the Agreements.

Investment Performance of the Manager, the Sub-Adviser and the Fund. Throughout the year, the Manager and the Sub-Adviser provided information on the investment performance of the Fund, including comparative performance information. The Board also reviewed information, prepared by the Manager, the Sub-Adviser and the independent consultant, comparing the Fund’s historical performance to relevant market indices and to the performance of other retail mid-cap blend funds. The Board noted that the Fund outperformed its performance category median during the three- and five-year periods, but underperformed its performance category median for the one- and ten-year periods. The Board noted that as of year-end 2013, the Fund had not yet transitioned to a pure mid-cap blend style. The Board considered the Fund’s strong performance since the transition to a pure mid-cap blend style, noting that the Fund ranked in the 33rd percentile as of April 30, 2014.

Costs of Services by the Manager. The Board reviewed the fees paid to the Manager and the Sub-Adviser and the other expenses borne by the Fund. The Board noted that the Manager, not the Fund, pays the Sub-Adviser’s fee under the sub-advisory agreement. The independent consultant provided comparative data in regard to the fees and expenses of the Fund and other retail mid-cap blend funds with comparable asset levels and distribution features. The Board noted that the Fund’s contractual management fee and its total expenses were below that of its peer group median and category median and that the Fund’s contractual management fees and total expenses both rank in the first quintile. Within the total asset range of $2 billion to $5 billion, the Fund’s effective management fee rate was lower than its peer group median and category median.

Economies of Scale and Profits Realized by the Manager and Sub-Adviser. The Board considered information regarding the Managers’ costs in serving as the Fund’s investment adviser and sub-adviser, including the costs associated with the personnel and systems necessary to manage the Fund, and information regarding the Managers’ profitability from their relationship with the Fund. The Board reviewed whether the Managers may realize economies of scale in managing and supporting the Fund. The Board noted that the Fund currently has management fee breakpoints, which are intended to share with Fund shareholders economies of scale that may exist as the Fund’s assets grow.

37       OPPENHEIMER MAIN STREET MID CAP FUND

BOARD APPROVAL OF THE FUND’S INVESTMENT ADVISORY

AND SUB-ADVISORY AGREEMENTS Unaudited / Continued

Other Benefits to the Managers. In addition to considering the profits realized by the Managers, the Board considered information that was provided regarding the direct and indirect benefits the Managers receive as a result of their relationship with the Fund, including compensation paid to the Managers’ affiliates and research provided to the Adviser in connection with permissible brokerage arrangements (soft dollar arrangements).The Board also considered that the Managers must be able to pay and retain experienced professional personnel at competitive rates to provide quality services to the Fund.

Conclusions. These factors were also considered by the independent Trustees meeting separately from the full Board, assisted by experienced counsel to the Fund and to the independent Trustees. Fund counsel and the independent Trustees’ counsel are independent of the Managers within the meaning and intent of the Securities and Exchange Commission Rules.

Based on its review of the information it received and its evaluations described above, the Board, including a majority of the independent Trustees, decided to continue the Agreements through August 31, 2015. In arriving at its decision, the Board did not identify any factor or factors as being more important than others, but considered all of the above information, and considered the terms and conditions of the Agreements, including the management fees, in light of all the surrounding circumstances.

38       OPPENHEIMER MAIN STREET MID CAP FUND

PORTFOLIO PROXY VOTING POLICIES AND PROCEDURES;

UPDATES TO STATEMENTS OF INVESTMENTS Unaudited

The Fund has adopted Portfolio Proxy Voting Policies and Procedures under which the Fund votes proxies relating to securities (“portfolio proxies”) held by the Fund. A description of the Fund’s Portfolio Proxy Voting Policies and Procedures is available (i) without charge, upon request, by calling the Fund toll-free at 1.800.CALL OPP (225.5677), (ii) on the Fund’s website at www.oppenheimerfunds.com, and (iii) on the SEC’s website at www.sec.gov. In addition, the Fund is required to file Form N-PX, with its complete proxy voting record for the 12 months ended June 30th, no later than August 31st of each year. The Fund’s voting record is available (i) without charge, upon request, by calling the Fund toll-free at 1.800.CALL OPP (225.5677), and (ii) in the Form N-PX filing on the SEC’s website at www.sec.gov.

The Fund files its complete schedule of portfolio holdings with the SEC for the first quarter and the third quarter of each fiscal year on Form N-Q. The Fund’s Form N-Q filings are available on the SEC’s website at www.sec.gov. Those forms may be reviewed and copied at the SEC’s Public Reference Room in Washington, D.C. Information on the operation of the Public Reference Room may be obtained by calling 1-800-SEC-0330.

Householding—Delivery of Shareholder Documents

This is to inform you about OppenheimerFunds’ “householding” policy. If more than one member of your household maintains an account in a particular fund, OppenheimerFunds will mail only one copy of the fund’s prospectus (or, if available, the fund’s summary prospectus), annual and semiannual report and privacy policy. The consolidation of these mailings, called householding, benefits your fund through reduced mailing expense, and benefits you by reducing the volume of mail you receive from OppenheimerFunds. Householding does not affect the delivery of your account statements.

Please note that we will continue to household these mailings for as long as you remain an OppenheimerFunds shareholder, unless you request otherwise. If you prefer to receive multiple copies of these materials, please call us at 1.800.CALL-OPP (225-5677). You may also notify us in writing or via email. We will begin sending you individual copies of the prospectus (or, if available, the summary prospectus), reports and privacy policy within 30 days of receiving your request to stop householding.

39       OPPENHEIMER MAIN STREET MID CAP FUND

OPPENHEIMER MAIN STREET MID CAP FUND

Trustees and Officers	Sam Freedman, Chairman of the Board of Trustees and Trustee
	Jon S. Fossel, Trustee
	Richard F. Grabish, Trustee
	Beverly L. Hamilton, Trustee
	Victoria J. Herget, Trustee
	Robert J. Malone, Trustee
	F. William Marshall, Jr., Trustee
	Karen L. Stuckey, Trustee
	James D. Vaughn, Trustee
	William F. Glavin, Jr., Trustee, President and Principal Executive Officer
	Raymond Anello, Vice President
	Matthew P. Ziehl, Vice President
	Raman Vardharaj, Vice President
	Joy Budzinski, Vice President
	Kristin Ketner, Vice President
	Magnus Krantz, Vice President
	Adam Weiner, Vice President
	Arthur P. Steinmetz, President and Principal Executive Officer
	Arthur S. Gabinet, Secretary and Chief Legal Officer
	Jennifer Sexton, Vice President and Chief Business Officer
	Mary Ann Picciotto, Chief Compliance Officer and Chief Anti-Money
	Laundering Officer
	Brian W. Wixted, Treasurer and Principal Financial & Accounting Officer
Manager	OFI Global Asset Management, Inc.
Sub-Adviser	OppenheimerFunds, Inc.
Distributor	OppenheimerFunds Distributor, Inc.
Transfer and Shareholder Servicing Agent	OFI Global Asset Management, Inc.
Sub-Transfer Agent	Shareholder Services, Inc. DBA OppenheimerFunds Services
Independent Registered Public Accounting Firm	KPMG LLP
Counsel	K&L Gates LLP
	The financial statements included herein have been taken from the records of the Fund without examination of those records by the independent registered public accounting firm.

© 2015 OppenheimerFunds, Inc. All rights reserved.

40       OPPENHEIMER MAIN STREET MID CAP FUND

PRIVACY POLICY NOTICE

As an Oppenheimer fund shareholder, you are entitled to know how we protect your personal information and how we limit its disclosure.

Information Sources

We obtain nonpublic personal information about our shareholders from the following sources:

— Applications or other forms

— When you create a user ID and password for online account access

— When you enroll in eDocs Direct, our electronic document delivery service

— Your transactions with us, our affiliates or others

— A software program on our website, often referred to as a “cookie,” which indicates which parts of our site you’ve visited

— When you set up challenge questions to reset your password online

If you visit oppenheimerfunds.com and do not log on to the secure account information areas, we do not obtain any personal information about you. When you do log on to a secure area, we do obtain your user ID and password to identify you. We also use this information to provide you with products and services you have requested, to inform you about products and services that you may be interested in and assist you in other ways.

We do not collect personal information through our website unless you willingly provide it to us, either directly by email or in those areas of the website that request information. In order to update your personal information (including your mailing address, email address and phone number) you must first log on and visit your user profile.

If you have set your browser to warn you before accepting cookies, you will receive the warning message with each cookie. You can refuse cookies by turning them off in your browser. However, doing so may limit your access to certain sections of our website.

We use cookies to help us improve and manage our website. For example, cookies help us recognize new versus repeat visitors to the site, track the pages visited, and enable some special features on the website. This data helps us provide a better service for our website visitors.

Protection of Information

We do not disclose any non-public personal information (such as names on a customer list) about current or former customers to anyone, except as permitted by law.

Disclosure of Information

Copies of confirmations, account statements and other documents reporting activity in your fund accounts are made available to your financial advisor (as designated by you). We may also use details about you and your investments to help us, our financial service affiliates, or firms that jointly market their financial products and services with ours, to better serve your investment needs or suggest financial services or educational material that may be of interest to you. If this requires us to provide you with an opportunity to “opt in” or “opt out” of such information sharing with a firm not affiliated with us, you will receive notification on how to do so, before any such sharing takes place.

Right of Refusal

We will not disclose your personal information to unaffiliated third parties (except as permitted by law), unless we first offer you a reasonable opportunity to refuse or “opt out” of such disclosure.

41       OPPENHEIMER MAIN STREET MID CAP FUND

PRIVACY POLICY NOTICE Continued

Internet Security and Encryption

In general, the email services provided by our website are encrypted and provide a secure and private means of communication with us. To protect your own privacy, confidential and/or personal information should only be communicated via email when you are advised that you are using a secure website.

As a security measure, we do not include personal or account information in non-secure emails, and we advise you not to send such information to us in non-secure emails. Instead, you may take advantage of the secure features of our website to encrypt your email correspondence. To do this, you will need to use a browser that supports Secure Sockets Layer (SSL) protocol.

We do not guarantee or warrant that any part of our website, including files available for download, are free of viruses or other harmful code. It is your responsibility to take appropriate precautions, such as use of an anti-virus software package, to protect your computer hardware and software.

— All transactions, including redemptions, exchanges and purchases, are secured by SSL and 128-bit encryption. SSL is used to establish a secure connection between your PC and OppenheimerFunds’ server. It transmits information in an encrypted and scrambled format.

— Encryption is achieved through an electronic scrambling technology that uses a “key” to code and then decode the data. Encryption acts like the cable converter box you may have on your television set. It scrambles data with a secret code so that no one can make sense of it while it is being transmitted. When the data reaches its destination, the same software unscrambles the data.

— You can exit the secure area by either closing your browser, or for added security, you can use the Log Out button before you close your browser.

Other Security Measures

We maintain physical, electronic and procedural safeguards to protect your personal account information. Our employees and agents have access to that information only so that they may offer you products or provide services, for example, when responding to your account questions.

How You Can Help

You can also do your part to keep your account information private and to prevent unauthorized transactions. If you obtain a user ID and password for your account, do not allow it to be used by anyone else. Also, take special precautions when accessing your account on a computer used by others.

Who We Are

This joint notice describes the privacy policies of the Oppenheimer funds, OppenheimerFunds, Inc., each of its investment adviser subsidiaries, OppenheimerFunds Distributor, Inc. and OFI Global Trust Co. It applies to all Oppenheimer fund accounts you presently have, or may open in the future, using your Social Security number—whether or not you remain a shareholder of our funds. This notice was last updated November 2014. In the event it is updated or changed, we will post an updated notice on our website at oppenheimerfunds.com. If you have any questions about this privacy policy, write to us at P.O. Box 5270, Denver, CO 80217-5270, email us by clicking on the Contact Us section of our website at oppenheimerfunds.com or call us at 1.800.CALL OPP (225.5677).

42       OPPENHEIMER MAIN STREET MID CAP FUND

THIS PAGE INTENTIONALLY LEFT BLANK.

43       OPPENHEIMER MAIN STREET MID CAP FUND

THIS PAGE INTENTIONALLY LEFT BLANK.

44       OPPENHEIMER MAIN STREET MID CAP FUND

THIS PAGE INTENTIONALLY LEFT BLANK.

45       OPPENHEIMER MAIN STREET MID CAP FUND

THIS PAGE INTENTIONALLY LEFT BLANK.

46       OPPENHEIMER MAIN STREET MID CAP FUND

THIS PAGE INTENTIONALLY LEFT BLANK.

47       OPPENHEIMER MAIN STREET MID CAP FUND

Item 2. Code of Ethics.

Not applicable to semiannual reports.

Item 3. Audit Committee Financial Expert.

Not applicable to semiannual reports.

Item 4. Principal Accountant Fees and Services.

Not applicable to semiannual reports.

Item 5. Audit Committee of Listed Registrants

Not applicable.

Item 6. Schedule of Investments.

a) Not applicable. The complete schedule of investments is included in Item 1 of this Form N-CSR.

b) Not applicable.

Item 7. Disclosure of Proxy Voting Policies and Procedures for Closed-End Management Investment Companies.

Not applicable.

Item 8. Portfolio Managers of Closed-End Management Investment Companies.

Not applicable.

Item 9. Purchases of Equity Securities by Closed-End Management Investment Company and Affiliated Purchasers.

Not applicable.

Item 10. Submission of Matters to a Vote of Security Holders.

The Fund’s Governance Committee Provisions with Respect to Nominations of Directors/Trustees to the Respective Boards

None

Item 11. Controls and Procedures.

Based on their evaluation of the registrant’s disclosure controls and procedures (as defined in rule 30a-3(c) under the Investment Company Act of 1940 (17 CFR 270.30a-3(c)) as of 12/31/2014, the registrant’s principal executive officer and principal financial officer found the registrant’s disclosure controls and procedures to provide reasonable assurances that information required to be disclosed by the registrant in the reports that it files under the Securities Exchange Act of 1934 (a) is accumulated and communicated to registrant’s management, including its principal executive officer and principal financial officer, to allow timely decisions regarding required disclosure, and (b) is recorded, processed, summarized and reported, within the time

periods specified in the rules and forms adopted by the U.S. Securities and Exchange Commission.

There have been no changes in the registrant’s internal controls over financial reporting that occurred during the registrant’s second fiscal quarter of the period covered by this report that have materially affected, or are reasonably likely to materially affect, the registrant’s internal control over financial reporting.

Item 12. Exhibits.

(a) (1) Not applicable to semiannual reports.

(2) Exhibits attached hereto.

(3) Not applicable.

(b) Exhibit attached hereto.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934 and the Investment Company Act of 1940, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

Oppenheimer Main Street Mid Cap Fund

By:	/s/ Arthur P. Steinmetz
	Arthur P. Steinmetz
	Principal Executive Officer
Date:	2/11/2015

Pursuant to the requirements of the Securities Exchange Act of 1934 and the Investment Company Act of 1940, this report has been signed below by the following persons on behalf of the registrant and in the capacities and on the dates indicated.

By:	/s/ Arthur P. Steinmetz
	Arthur P. Steinmetz
	Principal Executive Officer
Date:	2/11/2015

By:	/s/ Brian W. Wixted
	Brian W. Wixted
	Principal Financial Officer
Date:	2/11/2015

EX-99.CERT

Exhibit 99.CERT

Section 302 Certifications

CERTIFICATIONS

I, Arthur P. Steinmetz, certify that:

1. I have reviewed this report on Form N-CSR of Oppenheimer Main Street Mid Cap Fund;

2. Based on my knowledge, this report does not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements made, in light of the circumstances under which such statements were made, not misleading with respect to the period covered by this report;

3. Based on my knowledge, the financial statements, and other financial information included in this report, fairly present in all material respects the financial condition, results of operations, changes in net assets, and cash flows (if the financial statements are required to include a statement of cash flows) of the registrant as of, and for, the periods presented in this report;

4. The registrant’s other certifying officer and I are responsible for establishing and maintaining disclosure controls and procedures (as defined in Rule 30a-3(c) under the Investment Company Act of 1940) and internal control over financial reporting (as defined in Rule 30a-3(d) under the Investment Company Act of 1940) for the registrant and have:

(a) Designed such disclosure controls and procedures, or caused such disclosure controls and procedures to be designed under our supervision, to ensure that material information relating to the registrant, including its consolidated subsidiaries, is made known to us by others within those entities, particularly during the period in which this report is being prepared;

(b) Designed such internal control over financial reporting, or caused such internal control over financial reporting to be designed under our supervision, to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with generally accepted accounting principles;

(c) Evaluated the effectiveness of the registrant’s disclosure controls and procedures and presented in this report our conclusions about the effectiveness of the disclosure controls and procedures, as of a date within 90 days prior to the filing date of this report based on such evaluation; and

(d) Disclosed in this report any change in the registrant’s internal control over financial reporting that occurred during the second fiscal quarter of the period covered by this report that has materially affected, or is reasonably likely to materially affect, the registrant’s internal control over financial reporting; and

5. The registrant’s other certifying officer and I have disclosed to the registrant’s auditors and the audit committee of the registrant’s board of Trustees (or persons performing the equivalent functions):

(a) All significant deficiencies and material weaknesses in the design or operation of internal control over financial reporting which are reasonably likely to adversely affect the registrant’s ability to record, process, summarize, and report financial information; and

(b) Any fraud, whether or not material, that involves management or other employees who have a significant role in the registrant’s internal control over financial reporting.

Date: 2/11/2015

/s/ Arthur P. Steinmetz

Arthur P. Steinmetz

Principal Executive Officer

Exhibit 99.CERT

Section 302 Certifications

CERTIFICATIONS

I, Brian W. Wixted, certify that:

1. I have reviewed this report on Form N-CSR of Oppenheimer Main Street Mid Cap Fund;

2. Based on my knowledge, this report does not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements made, in light of the circumstances under which such statements were made, not misleading with respect to the period covered by this report;

3. Based on my knowledge, the financial statements, and other financial information included in this report, fairly present in all material respects the financial condition, results of operations, changes in net assets, and cash flows (if the financial statements are required to include a statement of cash flows) of the registrant as of, and for, the periods presented in this report;

4. The registrant’s other certifying officer and I are responsible for establishing and maintaining disclosure controls and procedures (as defined in Rule 30a-3(c) under the Investment Company Act of 1940) and internal control over financial reporting (as defined in Rule 30a-3(d) under the Investment Company Act of 1940) for the registrant and have:

(a) Designed such disclosure controls and procedures, or caused such disclosure controls and procedures to be designed under our supervision, to ensure that material information relating to the registrant, including its consolidated subsidiaries, is made known to us by others within those entities, particularly during the period in which this report is being prepared;

(b) Designed such internal control over financial reporting, or caused such internal control over financial reporting to be designed under our supervision, to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with generally accepted accounting principles;

(c) Evaluated the effectiveness of the registrant’s disclosure controls and procedures and presented in this report our conclusions about the effectiveness of the disclosure controls and procedures, as of a date within 90 days prior to the filing date of this report based on such evaluation; and

(d) Disclosed in this report any change in the registrant’s internal control over financial reporting that occurred during the second fiscal quarter of the period covered by this report that has materially affected, or is reasonably likely to materially affect, the registrant’s internal control over financial reporting; and

5. The registrant’s other certifying officer and I have disclosed to the registrant’s auditors and the audit committee of the registrant’s board of Trustees (or persons performing the equivalent functions):

(a) All significant deficiencies and material weaknesses in the design or operation of internal control over financial reporting which are reasonably likely to adversely affect the registrant’s ability to record, process, summarize, and report financial information; and

(b) Any fraud, whether or not material, that involves management or other employees who have a significant role in the registrant’s internal control over financial reporting.

Date: 2/11/2015

/s/ Brian W. Wixted

Brian W. Wixted

Principal Financial Officer

EX-99.906CERT

EX-99.906CERT

Section 906 Certifications

CERTIFICATION PURSUANT TO 18 U.S.C SECTION 1350,

AS ADOPTED PURSUANT TO

SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

Arthur P. Steinmetz, Principal Executive Officer, and Brian W. Wixted, Principal Financial Officer, of Oppenheimer Main Street Mid Cap Fund (the “Registrant”), each certify to the best of his knowledge that:

1. The Registrant’s periodic report on Form N-CSR for the period ended 12/31/2014 (the “Form N-CSR”) fully complies with the requirements of Section 15(d) of the Securities Exchange Act of 1934, as amended; and

2. The information contained in the Form N-CSR fairly presents, in all material respects, the financial condition and results of operations of the Registrant. This certification is being furnished to the Commission solely pursuant to 18 U.S.C. § 1350 and is not being filed as part of the Form N-CSR filed with the Commission.

Principal Executive Officer	Principal Financial Officer
Oppenheimer Main Street Mid Cap Fund	Oppenheimer Main Street Mid Cap Fund
/s/ Arthur P. Steinmetz	/s/ Brian W. Wixted
Arthur P. Steinmetz	Brian W. Wixted
Date: 2/11/2015	Date: 2/11/2015