- O'Fallon (Population: 46,169)
- Oak Grove (Population: 5,535)
- Oakland (Population: 1,540)
- Oakville (Population: 35,309)
- Odessa (Population: 4,818)
- Olivette (Population: 7,438)
- Oran (Population: 1,264)
- Osage Beach (Population: 3,662)
- Overland (Population: 16,838)
- Owensville (Population: 2,500)
- Ozark (Population: 9,665)
