Ohio frost dates: O
| City | Last frost | First frost | Season | Zone |
|---|---|---|---|---|
| Oak Harbor | Apr 26 | Oct 23 | 180 d | 6b |
| Oak Hill | Apr 22 | Oct 21 | 182 d | 6b |
| Oakwood | Apr 25 | Oct 19 | 177 d | 6a |
| Oberlin | Apr 27 | Oct 22 | 178 d | 6b |
| Oceola | Apr 19 | Oct 27 | 191 d | 6a |
| Ohio City | Apr 21 | Oct 26 | 188 d | 6a |
| Okeana | Apr 19 | Oct 22 | 186 d | 6b |
| Okolona | Apr 23 | Oct 24 | 184 d | 6a |
| Old Fort | Apr 21 | Oct 24 | 186 d | 6a |
| Old Washington | Apr 28 | Oct 17 | 172 d | 6b |
| Olmsted Falls | Apr 20 | Nov 3 | 197 d | 6b |
| Ontario | Apr 26 | Oct 22 | 179 d | 6a |
| Orangeville | May 1 | Oct 17 | 169 d | 6a |
| Oregon | Apr 20 | Oct 27 | 190 d | 6b |
| Oregonia | Apr 17 | Oct 22 | 188 d | 6b |
| Orient | Apr 23 | Oct 19 | 179 d | 6b |
| Orrville | Apr 29 | Oct 14 | 168 d | 6a |
| Orwell | May 10 | Oct 10 | 153 d | 6a |
| Osgood | Apr 27 | Oct 19 | 175 d | 6a |
| Ostrander | Apr 19 | Oct 26 | 190 d | 6b |
| Ottawa | Apr 23 | Oct 22 | 182 d | 6a |
| Ottoville | Apr 21 | Oct 26 | 188 d | 6a |
| Otway | Apr 21 | Oct 24 | 186 d | 6b |
| Overpeck | Apr 22 | Oct 18 | 179 d | 6b |
| Owensville | Apr 23 | Oct 18 | 178 d | 6b |
| Oxford | Apr 19 | Oct 22 | 186 d | 6a |