- O'connell Island
- O'Leary
- O'Neill
- Oakdale
- Oakfield
- Oakland
- Oakland Acres
- Oakland Mills
- Oakley
- Oakville
- Oakwood
- Oakwood Heights Mobile Home Park
- Oasis
- Ocheyedan
- Odebolt
- Oelwein
- Ogden
- Okoboji
- Olaf
- Old Balltown
- Old Orchard Mobile Home Park
- Old Peru
- Old Town
- Old Tripoli
- Olds
- Olin
- Olivet
- Ollie
- Olmitz
- Onawa