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 2020 census, O'Fallon had a population of 91,316, 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. The two O'Fallons are one of the few pairs of same-named municipalities to be part of the same MSA. History O'Fallon was founded in 1856 by Nicholas Krekel. The community was named by Krekel's older brother, Judge Arnold Krekel, after John O'Fallon, the president of the North Missouri Railroad. A post office called O'Fallon has been in operation since 1859 with its first postmaster being Nicholas Krekel. The St. Mary's Institute of O'Fallon was listed on the National Register of Historic Places in 2007. In 2006, Money magazine named O'Fallon 39th in its "Best 100 Places to Live". The magazine also ranked O'Fallon 68th out of 100 in 2008, 26th out of 100 in 2010, and 42nd out of 100 in 2017. Geography According to the United States Census Bureau, the city has a total area of , of which are land and is covered by water. Demographics 2010 census As of the census of 2010, 79,329 people, 28,234 households, and 21,436 families were residing in the city. The population density was . The 29,376 housing units averaged . The racial makeup of the city was 89.9% White, 4.0% African American, 0.2% Native American, 3.2% Asian, 0.9% from other races, and 1.8% from two or more races. Hispanics or Latinos of any race were 2.7% of the population. Of the 28,234 households, 44.7% had children under 18 living with them, 61.6% were married couples living together, 10.0% had a female householder with no husband present, 4.3% had a male householder with no wife present, and 24.1% were not families. About 19.3% of all households were made up of individuals, and 6.2% had someone living alone who was 65 or older. The average household size was 2.80, and the average family size was 3.23. The median age in the city was 34.3 years. The age distribution of the city was 30% under 18; 7.1%