O'Fallon is located 14 miles east of St. Louis, Missouri in St. Clair County. Nearby Scott Air Force Base has been stimulating local businesses which provide many of its goods and services, it has also been a large provider of jobs in the civilian market. Scott has been a major contributor to the area home building market, one of O'Fallon's largest industries. A 2006 real estate survey has estimated the population of O'Fallon, Illinois at 25,822.