High Quality Content by WIKIPEDIA articles! 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. As of the 2010 census, the city had a total population of 28,281. The city is the third largest city in the Metro-East 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.