14 cities
| City | County | 2026 Pop. ↓ |
|---|---|---|
| Orono | Penobscot County | 12,217 |
| Old Orchard Beach | York County | 9,544 |
| Old Town | Penobscot County | 7,617 |
| Oakland | Kennebec County | 6,351 |
| Oxford | Oxford County | 4,403 |
| Orrington | Penobscot County | 3,929 |
| Orland | Hancock County | 2,210 |
| Otisfield | Oxford County | 1,863 |
| Ogunquit | York County | 1,601 |
| Owls Head | Knox County | 1,471 |
| Otis | Hancock County | 660 |
| Oakfield | Aroostook County | 651 |
| Orient | Aroostook County | 150 |
| Osborn | Hancock County | 63 |