14 cities
| City | County | 2025 Pop. ↓ |
|---|---|---|
| Orono | Penobscot County | 12,259 |
| Old Orchard Beach | York County | 9,501 |
| Old Town | Penobscot County | 7,389 |
| Oakland | Kennebec County | 6,413 |
| Oxford | Oxford County | 4,442 |
| Orrington | Penobscot County | 3,922 |
| Orland | Hancock County | 2,203 |
| Otisfield | Oxford County | 1,904 |
| Ogunquit | York County | 1,597 |
| Owls Head | Knox County | 1,478 |
| Oakfield | Aroostook County | 666 |
| Otis | Hancock County | 665 |
| Orient | Aroostook County | 153 |
| Osborn | Hancock County | 66 |