Red Oak is a city in, and the county seat of, Montgomery County, Iowa, United States, located along the East Nishnabotna River. The popula…Red Oak is a city in, and the county seat of, Montgomery County, Iowa, United States, located along the East Nishnabotna River. The population was 5,596 in the 2020 census, a decline from the 6,197 population in 2000.