Maine frost dates: O
| City | Last frost | First frost | Season | Zone |
|---|---|---|---|---|
| Oakfield | May 19 | Sep 23 | 127 d | 4b |
| Oakland | May 2 | Oct 10 | 161 d | 5a |
| Ocean Park | May 9 | Oct 3 | 147 d | 6a |
| Ogunquit | May 1 | Oct 14 | 166 d | 6a |
| Old Orchard Beach | Apr 26 | Oct 15 | 172 d | 6a |
| Old Town | May 13 | Sep 28 | 138 d | 5a |
| Oquossoc | May 23 | Sep 24 | 124 d | 4b |
| Orient | May 25 | Sep 24 | 122 d | 4b |
| Orland | May 8 | Oct 9 | 154 d | 5b |
| Orono | May 13 | Sep 28 | 138 d | 5a |
| Orrington | May 4 | Oct 4 | 153 d | 5a |
| Orrs Island | May 2 | Oct 10 | 161 d | 6a |
| Owls Head | Apr 27 | Oct 25 | 181 d | 6a |
| Oxbow | May 13 | Sep 30 | 140 d | 4b |
| Oxford | May 3 | Oct 7 | 157 d | 5b |