Osceola is a city in St. Clair County, Missouri, United States. The population was 835 at the 2000 census. It is the county seat of St. Clair County. The town was the site of the September 1861 Sacking of Osceola by Jayhawkers in which the town was burned and courthouse looted. The event inspired the 1976 Clint Eastwood film The Outlaw Josey Wales.