Osceola is a city in and the county seat of St. Clair County, Missouri, United States. The population was 909 at the 2020 census. During th…Osceola is a city in and the county seat of St. Clair County, Missouri, United States. The population was 909 at the 2020 census. During the American Civil War, Osceola was the site of the Sacking of Osceola.