O'Fallon is a city in St. Clair County, Illinois, United States. The 2010 census listed the population at 28,281. The city is the third largest city in the Metro-East region and Southern Illinois. Scott Air Force Base is nearby.
Like its namesake in St. Charles County, Missouri, O'Fallon is part of the St. Louis Metropolitan Statistical Area. This makes O'Fallon (along with the two Troys in Illinois and Missouri) one of the few pairs of like-named municipalities to be part of the same MSA.