14 cities
| City | County | 2025 Pop. ↓ |
|---|---|---|
| Orono | Penobscot County | 12,208 |
| Old Orchard Beach | York County | 9,491 |
| Old Town | Penobscot County | 7,564 |
| Oakland | Kennebec County | 6,351 |
| Oxford | Oxford County | 4,401 |
| Orrington | Penobscot County | 3,912 |
| Orland | Hancock County | 2,220 |
| Otisfield | Oxford County | 1,874 |
| Ogunquit | York County | 1,601 |
| Owls Head | Knox County | 1,487 |
| Otis | Hancock County | 664 |
| Oakfield | Aroostook County | 655 |
| Orient | Aroostook County | 151 |
| Osborn | Hancock County | 64 |