O'Fallon is a city in St. Clair County, Illinois, United States. The 2020 census listed the population at 32,289. The city is the second largest city in the Metro East region of the St. Louis metropolitan area and Southern Illinois. It sits 5 miles from Scott Air Force Base and 18 miles from Downtown St. Louis. Like its namesake in St. Charles County, Missouri, O'Fallon is part of the St. Louis metro area.