Oskaloosa is a city in, and the county seat of, Mahaska County, Iowa. The population was 11,558 in the 2020 U.S. census. In the late nineteenth and early twentieth century, Oskaloosa was a national center of bituminous coal mining. Today, Oskaloosa is home to William Penn University, a private university, and Lake Keomah State Park which is located four miles east of the city.