- Oleum
- Olga
- Olinda
- Olive Hill
- Olivehurst
- Olympia
- Olympic Valley
- Omega
- Omira
- Omo Ranch
- Omus
- One Hundred Palms
- O'Neals
- Ono
- Ontario
- Onyx
- Opal Cliffs
- Ophir
- Ophir Hill Acres
- Ora
- Orange
- Orange Avenue Junction
- Orange Cove
- Orange Mobile Home Park
- Orange Park Acres
- Orangevale
- Orchams
- Orchard
- Orchard Lane Mobile Home Park
- Orcutt