Cromwell is a town in Seminole County, Oklahoma, United States. It is within the jurisdiction of the Muscogee Nation. The population was 238 at the time of the 2020 census. It was named for oilman Joe I. Cromwell, who platted the original town in 1923. The population soared to several thousand people in a few weeks, and lawlessness was rampant in the community. Retired legendary Old West lawman Bill Tilghman was hired as Town Marshal to restore order.