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 suburb of St. Louis, as well as 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. 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.” Money Magazine also ranked O’Fallon 68th out of 100 in 2008, 26th out of 100 in 2010, and 42nd out of 100 in 2017.According to the United States Census Bureau, the city has a total area of 29.20 square miles (75.63 km2), of which 29.19 square miles (75.60 km2) is land and 0.01 square miles (0.03 km2) is water.
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