O'Fallon (/oʊˈfælən/ oh-FAL-ən) is a city in St. Charles County, Missouri, United States. It is part of the St. Louis metropolitan statistical area, located along Interstates 64 and 70 between Lake St. Louis and St. Peters. As of the 2020 census, O'Fallon had a population of 91,316, making it the most populous suburb of St. Louis, as well as the most populous municipality in St. Charles County and the 7th most populous in Missouri. O'Fallon's namesake in St. Clair County, Illinois, is also part of the St. Louis region. The two cities are one of the few pairs of same-named municipalities to be part of the same metro area.

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 Nicholas Krekel as first postmaster. 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.

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) are land and 0.01 square miles (0.03 km2) is covered by water.

O'Fallon has a humid subtropical climate (Köppen Cfa). Summers are hot and humid, while winters are moderately cold.

The 2020 United States census counted 91,316 people, 31,681 households, and 23,754 families in O'Fallon. The population density was 2,994.0 per square mile (1,155.9/km2). There were 32,740 housing units at an average density of 1,073.4 per square mile (414.4/km2). The racial makeup was 82.28% (75,133) white, 5.06% (4,620) black or African-American, 0.22% (205) Native American, 4.06% (3,704) Asian, 0.1% (88) Pacific Islander, 1.46% (1,334) from other races, and 6.82% (6,232) from two or more races. Hispanic or Latino of any race was 3.5% (3,067) of the population.

Of the 31,681 households, 37.5% had children under the age of 18; 60.5% were married couples living together; 21.9% had a female householder with no husband present. Of all households, 19.9% consisted of individuals and 9.3% had someone living alone who was 65 years of age or older. The average household size was 2.8 and the average family size was 3.2.

25.4% of the population was under the age of 18, 6.8% from 18 to 24, 26.5% from 25 to 44, 26.2% from 45 to 64, and 11.6% who were 65 years of age or older. The median age was 37.5 years. For every 100 females, the population had 97.3 males. For every 100 females ages 18 and older, there were 93.2 males.

The 2016-2020 5-year American Community Survey estimates show that the median household income was $92,498 (with a margin of error of +/- $3,539) and the median family income was $106,419 (+/- $3,781). Males had a median income of $56,446 (+/- $2,450) versus $36,747 (+/- $2,327) for females. The median income for those above 16 years old was $45,900 (+/- $2,496). Approximately, 2.6% of families and 4.6% of the population were below the poverty line, including 5.1% of those under the age of 18 and 3.9% of those ages 65 or over.

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 2,717.7 inhabitants per square mile (1,049.3/km2). The 29,376 housing units averaged 1,006.4 per square mile (388.6/km2). 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% between 18 and 24; 30.8% from 25 to 44; 23.1% from 45 to 64; and 8.9% at 65 or older. The gender makeup of the city was 48.8% male and 51.2% female.

Mastercard has a major presence in O'Fallon. Venture Stores was headquartered and maintained a distribution center in O'Fallon, until its dissolution. The buildings are now occupied by True Manufacturing. 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. The Rascals played at CarShield Field in O'Fallon, which was built in 1999. It is located on Tom Ginnever Boulevard and T.R. Hughes Boulevard near downtown. The organization ceased operations after the 2019 season. The O'Fallon Hoots and the CarShield Collegiate League now play at the stadium.

O'Fallon operates under a charter form of government. The mayor serves four-year terms without term limits and is also the President of the City Council. The 10-member council consists of two members from each of the five wards; the council had 8 members until the fifth ward was created in 2010. City council members served two-year terms until 2010, when they switched to three-year terms. The council elects a President Pro Tempore from among its members, who presides over the council in the mayor's absence.

Bill Hennessy has served as mayor since 2009. The most senior member of the council is Debbie Cook, who was elected to a Ward 5 seat in a 2014 special election and was re-elected in 2016, 2019, and 2022. 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.

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. Ward 2 councilman Tom Herweck succeeded him as President Pro Tem.

O'Fallon is served mostly by the Fort Zumwalt School District. The westernmost part is served by the Wentzville R-IV School District. The south to southeastern part of the city is served by the Francis Howell R-III School District.

St. Dominic High School is a private Catholic school located in O'Fallon; Christian High School is a nondenominational Christian secondary school also located in O'Fallon.

Satellite campuses of Webster University and Lindenwood University are located in O'Fallon.

O'Fallon is served by the St. Charles City-County Library system, which has three branches in the area, two standard (Deer Run and Middendorf-Kredell) and one "express" location (Library Express at Winghaven).

While the City is within the jurisdiction of multiple agencies, fire protection is mostly provided by the O'Fallon Fire Protection District, which in 2007 became the first internationally accredited fire agency in Missouri. The award was made by the Center for Public Safety Excellence's Commission on Fire Accreditation International, which has approved accreditation status for only 120 fire agencies worldwide.