what county is o'fallon illinois in
O'Fallon, Illinois. O'Fallon is a city in St. Clair County, Illinois, United States, and one of the fastest-growing communities in the Metropolitan St. Louis area. A 2017 special census listed the city's population at 30,440. The city is the second largest city in the Metro-East region and Southern Illinois. Due to its close proximity to Scott Air Force Base, the population receives a boost from military and federal civilian personnel, defense contractors, and military retirees. The city center is located approxima