- Oakland (Population: 5,959)
- Ogunquit (Population: 1,226)
- Old Orchard Beach (Population: 8,856)
- Old Town (Population: 8,130)
- Orland (Population: 2,134)
- Orono (Population: 9,112)
- Orrington (Population: 3,526)
- Otisfield (Population: 1,560)
- Owls Head (Population: 1,601)
- Oxford (Population: 3,960)
