Crestview is the largest city and county seat of Okaloosa County, Florida, United States. The population was 27,134 at the 2020 census, up fro…Crestview is the largest city and county seat of Okaloosa County, Florida, United States. The population was 27,134 at the 2020 census, up from 20,978 at the 2010 census. With an elevation of 236 feet above sea level, it is one of the highest points in the state.