| City | County | 2026 Pop. ↓ |
|---|---|---|
| Orono | Penobscot County | 12,608 |
| Old Orchard Beach | York County | 9,585 |
| Old Town | Penobscot County | 7,661 |
| Oakland | Kennebec County | 6,350 |
| Oxford | Oxford County | 4,479 |
| Orrington | Penobscot County | 3,921 |
| Orland | Hancock County | 2,216 |
| Otisfield | Oxford County | 1,904 |
| Ogunquit | York County | 1,604 |
| Owls Head | Knox County | 1,488 |
| Otis | Hancock County | 664 |
| Oakfield | Aroostook County | 650 |
| Orient | Aroostook County | 149 |
| Osborn | Hancock County | 67 |