Duncan is a city in Stephens County, Oklahoma, United States. The population was 23,431 at the 2010 census. It is the county seat of Stephens County. The official birthdate of the town is considered to be when the first train arrived there on June 27, 1892. Statehood in 1907 turned the town into the county seat and a number of oil wells in Stephens County led to Duncan's rapid development in 1918.