O'Fallon Historical Museum houses six buildings of Fallon County History. We are open year round and admission is always free! Get a taste of what homestead life was like on the open prairie, tour the original Fallon County Jail, and see the World's Largest Steer (born and raised in Fallon County.)O'Fallon Historical Museum houses six buildings of Fallon County History. We are open year round and admission is always free! Get a taste of what homestead life was like on the open prairie, tour the original Fallon County Jail, and see the World's Largest Steer (born and raised in Fallon County.)