Washington frost dates: O
| City | Last frost | First frost | Season | Zone |
|---|---|---|---|---|
| Oak Harbor | Mar 13 | Nov 14 | 246 d | 8b |
| Oakesdale | May 7 | Sep 28 | 144 d | 6b |
| Oakville | May 1 | Oct 14 | 166 d | 8a |
| Ocean Park | Apr 6 | Oct 26 | 203 d | 9a |
| Ocean Shores | Mar 30 | Nov 14 | 229 d | 9a |
| Odessa | May 14 | Sep 26 | 135 d | 6b |
| Okanogan | Apr 30 | Oct 7 | 160 d | 7a |
| Olalla | Mar 11 | Nov 21 | 255 d | 8b |
| Olga | Feb 26 | Nov 29 | 276 d | 8b |
| Olympia | Apr 29 | Oct 14 | 168 d | 8b |
| Omak | Apr 30 | Oct 7 | 160 d | 6b |
| Onalaska | Apr 7 | Nov 13 | 220 d | 8b |
| Orcas | Mar 26 | Nov 10 | 229 d | 9a |
| Orient | May 8 | Oct 1 | 146 d | 6b |
| Orondo | May 14 | Oct 1 | 140 d | 7a |
| Oroville | Apr 26 | Oct 10 | 167 d | 6a |
| Orting | Apr 11 | Oct 30 | 202 d | 8b |
| Othello | Apr 30 | Oct 12 | 165 d | 7a |
| Otis Orchards | Apr 26 | Oct 15 | 172 d | 6b |
| Outlook | Apr 30 | Oct 8 | 161 d | 7b |
| Oysterville | Apr 6 | Oct 26 | 203 d | 9a |