Oakley is a city in Contra Costa County, California, United States. It is within the nine-county San Francisco Bay Area. The January 1, 2016 population was 40,144, as determined by the State Department of Finance. Oakley was incorporated in 1999 and is the most recently incorporated city in Contra Costa County. Oakley is part of the East Contra Costa Bicycle Plan, which has existing facilities in Oakley as well as plans for further expansion.