- Orwood
- Ozol
- Old Fort Jim
- Old Pino
- Oregon Bar (historical)
- Outingdale
- Ockenden
- Oilfields
- Old Bretz Mill
- Old Fig Garden
- Oleander
- Ora
- Oro Loma
- Oxalis
- Oak Leaf Island
- Ordbend
- Oak Glen
- Off Rock
- Obregon
- Orita
- Oteys Sierra Village
- Oil City
- Oil Junction
- Old Garlock
- Old River
- Old Town
- Omira
- Oak Hill
- Oban
- Ocean Park