Oak Park is a village in Cook County, Illinois, United States, adjacent to Chicago. It is the 26th-most populous municipality in Illinois, with a population of 54,318 as of the 2020 census. Oak Park was first settled in 1835 and later incorporated in 1902, when it separated from Cicero. It is closely tied to the smaller village of River Forest sharing a chamber of commerce and a high school, Oak Park and River Forest High School.