Wisconsin frost dates: O
| City | Last frost | First frost | Season | Zone |
|---|---|---|---|---|
| Oak Creek | Apr 22 | Oct 26 | 187 d | 6a |
| Oakdale | May 1 | Oct 3 | 155 d | 4b |
| Oakfield | May 1 | Oct 10 | 162 d | 5a |
| Oconomowoc | Apr 30 | Oct 12 | 165 d | 5b |
| Oconto | May 9 | Oct 6 | 150 d | 5a |
| Oconto Falls | May 9 | Oct 6 | 150 d | 5a |
| Odanah | May 23 | Oct 1 | 131 d | 5a |
| Ogdensburg | May 8 | Oct 6 | 151 d | 5a |
| Ogema | May 23 | Sep 23 | 123 d | 4a |
| Ojibwa | May 19 | Sep 27 | 131 d | 4a |
| Okauchee | Apr 30 | Oct 12 | 165 d | 5b |
| Omro | Apr 24 | Oct 20 | 179 d | 5b |
| Onalaska | Apr 23 | Oct 17 | 177 d | 5a |
| Oneida | May 1 | Oct 11 | 163 d | 5b |
| Ontario | Apr 27 | Oct 14 | 170 d | 4b |
| Oostburg | May 3 | Oct 16 | 166 d | 5b |
| Oregon | May 1 | Oct 9 | 161 d | 5a |
| Orfordville | Apr 29 | Oct 12 | 166 d | 5b |
| Osceola | May 4 | Oct 8 | 157 d | 4b |
| Oshkosh | Apr 24 | Oct 20 | 179 d | 5b |
| Osseo | May 7 | Oct 1 | 147 d | 4b |
| Owen | May 13 | Sep 26 | 136 d | 4b |
| Oxford | May 3 | Oct 5 | 155 d | 5a |