Sinclair Oil Corporation is an American petroleum corporation, founded by Harry F. Sinclair on May 1, 1916, as the Sinclair Oil and Refining Corporation by combining the assets of 11 small petroleum companies.

During September 1919, Harry F. Sinclair restructured Sinclair Oil and Refining Corporation, Sinclair Gulf Corporation, and 26 other related entities into Sinclair Consolidated Oil Corporation. In 1932, this new entity was renamed Consolidated Oil Corporation. In 1943, it was renamed Sinclair Oil Corporation.

Near the beginning of the Great Depression, Sinclair sold the remaining interest in its pipeline subsidiary to Standard Oil Company (Indiana) for US$72.5 million (Standard Oil had purchased a 50% interest in the pipeline subsidiary in 1921). With these funds, including an additional US$33.5 million from an additional common stock issue, Sinclair retired a number of promissory notes and prepared to weather the Depression with the remaining supply of cash.

During the Great Depression, Sinclair saved a number of other petroleum companies from receivership or bankruptcy, and acquired others to expand its operations. In 1932, Sinclair purchased the assets of Prairie Oil and Gas's pipeline and producing companies in the southern United States, and the Rio Grande Oil Company in California. The purchase of Prairie also gave Sinclair a 65% interest in Producers and Refiners Corporation (or Parco), which Sinclair subsequently acquired when Parco entered receivership in 1934. Lastly, in 1936, Sinclair purchased the East Coast marketing subsidiary of Richfield Oil Company, which had operated in receivership for several years. Richfield then reorganized, resulting in the creation of the Richfield Oil Corporation. Sinclair was instrumental in transferring capital and managerial assets into Richfield. Thirty years later, Richfield merged with Atlantic Refining, located on the East Coast, forming Atlantic Richfield.