O'Fallon
- County:
- St. Charles County
- County Seat:
- No
- Area (mi²):
- 30.989
- State:
- Missouri
O'Fallon is a city located in St. Charles County, Missouri. O'Fallon has a 2026 population of 95,355. O'Fallon is currently growing at a rate of 0.99% annually and its population has increased by 4.03% since the most recent census, which recorded a population of 91,665 in 2020.