- Oak Harbor (Population: 19,795)
- Ocean Park (Population: 1,459)
- Ocean Shores (Population: 3,836)
- Okanogan (Population: 2,484)
- Olympia (Population: 42,514)
- Omak (Population: 4,721)
- Opportunity (Population: 25,065)
- Orchards (Population: 17,852)
- Oroville (Population: 1,653)
- Orting (Population: 3,760)
- Othello (Population: 5,847)
- Otis Orchards-East Farms (Population: 6,318)
