Cromwell is a town in Seminole County, Oklahoma, United States. The population was 286 at the 2010 census, an 8 percent increase from 265 at the 2000 census. It was named for oilman Joe I. Cromwell, who platted the original town in 1923. The population exploded to a few 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. Tilghman was shot to death the following year, the most notable event in Cromwell's history.