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 municipality in St. Charles County and the seventh-largest in the state of Missouri. O'Fallon's namesake in St. Clair County, Illinois is also part of the St. Louis Metropolitan Statistical Area. The two O'Fallons are one of the few pairs of same-named municipalities to be part of the same MSA.