O'Fallon is a city located along Interstates 64 and 70 between Lake St. Louis and St. Peters in St. Charles County, Missouri, United States. It is part of the St. Louis metropolitan statistical area. As of the 2010 census, O'Fallon had a population of 79,329, making it the largest suburb of St. Louis, as well as the largest municipality in St. Charles County and the seventh-largest in Missouri. O'Fallon's namesake in St. Clair County, Illinois, is also part of the St. Louis metropolitan statistical area.