O
- Oakdale
- Oakland
- Oakland
- Oakland Acres
- Oakland Mills
- Oakley
- Oakville
- Oakwood
- Oakwood Heights Mobile Home Park
- Oasis
- Ocheyedan
- Odebolt
- Oelwein
- Ogden
- Okoboji
- Olaf
- Old Balltown
- Olds
- O'Leary
- Olin
- Olivet
- Ollie
- Olmitz
- Onawa
- Onawa Junction
- Oneida
- O'Neill
- Onslow
- Ontario
- Oralabor
- Oran
- Orange
- Orange City
- Orchard
- Ord
- Orient
- Orillia
- Orleans
- Ormanville
- Orson
- Orton
- Ortonville
- Osage
- Osborne
- Osceola
- Osgood
- Oskaloosa
- Ossian
- Osterdock
- Oswalt
- Otho
- Otley
- Oto
- Otranto
- Ottawa
- Ottawa
- Otter Creek
- Otterville
- Ottosen
- Ottumwa
- Ouandahl
- Owasa
- Owego
- Owen
- Oxford
- Oxford Junction
- Oxford Mills
- Oyens
- Ozark