- Octa
- Ohio City
- Oakshade
- Oceola
- Okeana
- Okolona
- Old Fort
- Olena
- Olive Branch
- Omega
- Oneida
- Oregonia
- Otsego
- Overpeck
- Olde West Chester
- Olmsted Falls
- Oregon
- Orrville
- Orwell
- Osgood
- Ostrander
- Ottawa Hills
- Ottoville
- Otway
- Owensville
- Oldtown Flats
- Osterholt Mobile Home Park
- Oakmont Creek
- Overlook Trailer Park
- Oak