Question: When was the firm who privatized OM Group after it was listed on the New York Stock Exchange founded?

Context paragraphs:
Apollo Global Management: Apollo Global Management, LLC is an American private equity firm, founded in 1990 by former Drexel Burnham Lambert banker Leon Black.  The firm specializes in leveraged buyout transactions and purchases of distressed securities involving corporate restructuring, special situations, and industry consolidations.  Apollo is headquartered in New York City, and also has offices in Purchase, New York, Los Angeles, Houston, London, Frankfurt, Luxembourg, Singapore, Hong Kong and Mumbai.

Buttonwood Agreement: The Buttonwood Agreement, which took place on May 17, 1792, started the New York Stock & Exchange Board now called the New York Stock Exchange.  This agreement was signed by 24 stockbrokers outside of 68 Wall Street New York under a buttonwood tree.  The organization drafted its constitution on March 8, 1817, and named itself the "New York Stock & Exchange Board".  In 1863, this name was shortened to its modern form, the "New York Stock Exchange".

Consolidated Stock Exchange of New York: The Consolidated Stock Exchange of New York, also known as the "New York Consolidated Stock Exchange" or "Consolidated", was a stock exchange in New York City, New York in direct competition to the New York Stock Exchange (NYSE) from 1885-1926.  It was formed from the merger of other smaller exchanges, and was referred to in the industry and press as the "Little Board."  By its official formation in 1885, its membership of 2403 was considered the second largest membership of any exchange in the United States.

Hamburg Stock Exchange: The Hamburg Stock Exchange (German: "Hamburger Börse" ) is a stock exchange founded in 1558 in the Free and Hanseatic city of Hamburg.  It is the oldest stock exchange in Germany.  Four different individual exchanges now exist under its umbrella: the Insurance Exchange, Grain Exchange, Coffee Exchange, along with the General Exchange.

Philadelphia Stock Exchange: Philadelphia Stock Exchange (PHLX), now known as NASDAQ OMX PHLX, is the oldest stock exchange in the United States.  It is now owned by The NASDAQ OMX Group.  Founded in 1790, the exchange was originally named the Board of Brokers of Philadelphia, also referred to as the Philadelphia Board of Brokers.  In 1875, the Board of Brokers changed its name to the Philadelphia Stock Exchange.  The exchange merged with the Baltimore Stock Exchange in 1949 and was named the Philadelphia-Baltimore Stock Exchange.  It merged with the Washington Stock Exchange in 1954, becoming known as the Philadelphia-Baltimore-Washington Stock Exchange (often abbreviated as PBW).  In 1969, the exchange acquired the Pittsburgh Stock Exchange.  In 2007, PHLX was purchased by NASDAQ for $652 million.

China Concepts Stock: China Concepts Stock is a set of stock of companies whose assets or earnings have significant activities in mainland China.  The People's Republic of China is undergoing major financial transformation, and many leading mainland-based companies have chosen to list themselves overseas to gain access to foreign investor capital.  Currently, there are China Concepts Stocks listed on several major stock exchanges around the globe, including the Hong Kong Stock Exchange (HKEx), Singapore Exchange (SGX), New York Stock Exchange (NYSE), NASDAQ, NYSE MKT (formerly known as the American Stock Exchange), London Stock Exchange (LSE), Euronext, and the Tokyo Stock Exchange (TSE).

American Stock Exchange Building: The American Stock Exchange Building, formerly known as the New York Curb Exchange Building, is the former headquarters of the American Stock Exchange.  It is located on Trinity Place in the Financial District of Lower Manhattan, New York City.  Built in 1921 and enlarged in 1929-31, the building represents a link to the historical practices of stock trading outside the strictures of the New York Stock Exchange, which took place outside ("on the curb") prior to the construction of this building.  It was designated a National Historic Landmark in 1978.  The building presently stands vacant, the stock exchange having moved out after merging with the NYSE in 2008.

Financial District, Manhattan: The Financial District, also known as FiDi, is a neighborhood located at the southern tip of the borough of Manhattan in New York City, which comprises the offices and headquarters of many of the city's major financial institutions, including the New York Stock Exchange and the Federal Reserve Bank of New York.  Anchored on Wall Street in the Financial District, New York City has been called both the most financially powerful city and the leading financial center of the world, and the New York Stock Exchange is the world's largest stock exchange by total market capitalization.  Several other major exchanges have or had headquarters in the Financial District, including the New York Mercantile Exchange, NASDAQ, the New York Board of Trade, and the former American Stock Exchange.

Nasdaq, Inc.: Nasdaq, Inc. is an American multinational financial services corporation that owns and operates (and is listed on) the NASDAQ (formerly National Association of Securities Dealers Automated Quotations) stock market and eight European stock exchanges, namely Armenian Stock Exchange, Copenhagen Stock Exchange, Helsinki Stock Exchange, Iceland Stock Exchange, Riga Stock Exchange, Stockholm Stock Exchange, Tallinn Stock Exchange, and NASDAQ OMX Vilnius.  It is headquartered in New York City, and its president and chief executive officer is Adena Friedman.

OM Group: OM Group Incorporated is a metal-based chemistry firm based in Cleveland, Ohio, United States.  It is a provider of speciality chemicals, advanced materials and technologies.  The company was listed on the New York Stock Exchange prior to being privatized by Apollo Global Management in June 2015.

Supporting paragraphs: OM Group Apollo Global Management
Answer: 1990