Simplify the following text for a Grade 5 native English-speaking person.
Original: Founded in 1854 , O'Fallon 's namesake comes from Colonel John O'Fallon who was a wealthy gentleman from St. Louis . In downtown O'Fallon , a Baltimore and Ohio Railroad railroad depot was built , which helped put O'Fallon on the map .
Simplified: O'Fallon is a city of Illinois in the United States .