Connecticut frost dates: O
| City | Last frost | First frost | Season | Zone |
|---|---|---|---|---|
| Oakdale | Apr 14 | Oct 24 | 193 d | 6b |
| Oakville | Apr 26 | Oct 16 | 173 d | 6b |
| Old Greenwich | Apr 17 | Oct 23 | 189 d | 7b |
| Old Lyme | Apr 16 | Oct 28 | 195 d | 7a |
| Old Mystic | Apr 8 | Nov 2 | 208 d | 7a |
| Old Saybrook | Apr 6 | Nov 17 | 225 d | 7a |
| Oneco | Apr 25 | Oct 19 | 177 d | 6b |
| Orange | Apr 25 | Oct 17 | 175 d | 7a |
| Oxford | Apr 25 | Oct 17 | 175 d | 6b |