- O'Neill
- Old Balltown
- Oakwood
- Oakwood Heights Mobile Home Park
- Oralabor
- Oakdale
- Oran
- Otley
- Otranto
- Otter Creek
- Overland Mobile Home Park
- Old Orchard Mobile Home Park
- Ontario
- Ormanville
- Ottumwa Junction
- Orilla
- Ottawa
- Orson
- Orton
- Ovia
- Oakland Mills
- Owego
- Olaf
- Ozark
- Oswalt
- Oasis
- Oakland Acres
- Ocheyedan
- Odebolt
- Oelwein