- Old Garlock
- Old Gilroy
- Old Hilltown
- Old Hopland
- Old Hulbert Place
- Old Mammoth
- Old Mill Park Mobile Home Park
- Old Ornbaun Hot Springs
- Old Pino
- Old Point Comfort
- Old Red Rock Place
- Old River
- Old Station
- Old Town
- Old Town
- Oleander
- Olema
- Oleum
- Olga
- Olinda
- Olive Hill
- Olympia
- Olympic Valley
- Omega
- Omira
- Omo Ranch
- Omus
- One Hundred Palms
- O'Neals
- Ono