Cleveland County is a county in the central part of the U.S. state of Oklahoma. The population was 269,340 at the 2013 census. It is the third most populous county in Oklahoma. Its county seat is Norman. Founded in 1890 and was named after President Grover Cleveland.
According to the U.S. Census Bureau, the county has a total area of 558 square miles (1,445.2 km), of which 536 square miles (1,388.2 km) is land and 22 square miles (57.0 km) (3.98%) is water. It is the seventh smallest county in the state.
Cleveland County contains the reservoir Lake Thunderbird 5,349 acres (21.65 km), constructed between 1962 and 1965.
Cleveland County is the origin of the Little River, a tributary of the Canadian River, 90 miles (140 km) long. The Canadian River defines the southern border of Cleveland County.