Onex Corporation (TSX: OCX) is a Toronto based private equity investment firm and holding company. Today it is a publicly traded company but founder Gerry Schwartz holds 67.6% of the voting control and continues to serve as Chairman and CEO. Founded in 1983, Onex is headquartered on the 49th floor of Brookfield Place in Toronto, with a branch office in New York City.
The firm invests in a wide array of industries. It has shown a particular interest in buying the high-cost manufacturing portion of companies and turning these into low cost suppliers. In the 1980s it played this role in the auto parts industry. In 1996 it bought IBM's manufacturing division Celestica. Although the firm has majority control over most of its subsidiaries, rarely does it have whole ownership.
Onex is involved in the American health care industry, owning a number of firms there. They are also in the movie theatre business, both in Canada with Cineplex Entertainment, as well as in Mexico with Cinemex, this latter one in partnership with U.S. based The Carlyle Group. The firm has also long been invested in the airline industry and is well known for its failed 1999 attempt to buy both Air Canada and Canadian Airlines to merge them.  In 2004 Onex bought Boeing Commercial Airplanes' Kansas and Oklahoma manufacturing facilities, forming Spirit AeroSystems, which was later partially floated in late 2006. In late 2006, Onex became a member of the Airline Partners Australia consortium in a takeover bid for Qantas. In January 2007, Onex signed a deal to acquire the Raytheon Aircraft Company from Raytheon (forming Hawker Beechcraft).
In January 2007, Onex signed a deal to acquire Eastman Kodak's medical imaging unit, including a Kodak factory in White City, Oregon. The company formed is now known as Carestream Health.
On June 28, 2007, Onex announced that it would partner with The Carlyle Group to buy the Allison Transmission unit of General Motors for $5.6 billion.
Other companies in which Onex owns a significant stake include, as of November 2008, Sitel, American Medical Response, EmCare, Jeld Wen, Husky Injection Molding Systems Ltd, Canadian Securities Institute and CiCi's Pizza.
On December 3, 2012, Onex took a 50% stake in aircraft lessor BBAM and invested $75 million in aircraft leasing firm FLY Leasing. 
In May 2013, Nielsen Holdings announced it would sell its expositions business to Onex Corporation for $950 million.