- Oak Grove
- Old Forbestown
- Oregon City
- Oroleve
- Oroville Junction
- Oak Grove
- Oak Park Estates
- Oak Run
- Oakville
- Oasis
- Oasis, Mono
- O'Brien
- Ocotillo Wells
- Ogilby
- Old Station
- Olema
- Omo Ranch
- O'Neals
- Ono
- Orange Park Acres
- Oregon House
- Orick
- Orleans
- Oro Grande
- Oroville East
- Otay
- Otterbein
- Owenyo
- Oleum
- Orinda Village