O'Fallon (/oʊˈfælən/ oh-FAL-ən) is a city in St. Charles County, Missouri, United States.

O'Fallon's namesake in St. Clair County, Illinois, is also part of the St. Louis region.

[6] A post office called O'Fallon has been in operation since 1859 with Nicholas Krekel as first postmaster.

[7] The St. Mary's Institute of O'Fallon was listed on the National Register of Historic Places in 2007.

The 2020 United States census[16] counted 91,316 people, 31,681 households, and 23,754 families in O'Fallon.

Venture Stores[22] was headquartered and maintained a distribution center in O'Fallon, until its dissolution.

Air Evac Lifeteam, a medical helicopter service for the rural areas of the Ozarks, moved its headquarters to O'Fallon in 2015.

O'Fallon was the home of the River City Rascals independent Frontier League baseball team.

In January 2022, city councilwoman Katie Gatewood became the first elected official in O'Fallon to be impeached and removed from office.

She was accused of impeding the duties of the police chief and lying to the Council about the identity of a whistleblower.

[23] As of January 2023, there is one vacancy on the council: Ward 1 councilman and President Pro Tem Dave Hinman resigned after being elected to the Missouri House of Representatives.

The south to southeastern part of the city is served by the Francis Howell R-III School District.

The western portion of the city is served by the Wentzville Fire Protection District.

[27] The eastern portion of the city is served by Central County Fire Rescue.