Iowa frost dates: O
| City | Last frost | First frost | Season | Zone |
|---|---|---|---|---|
| Oakland | Apr 23 | Oct 14 | 174 d | 5b |
| Oakville | Apr 14 | Oct 24 | 193 d | 5b |
| Ocheyedan | Apr 30 | Oct 10 | 163 d | 5a |
| Odebolt | Apr 27 | Oct 12 | 168 d | 5a |
| Oelwein | Apr 27 | Oct 13 | 169 d | 5a |
| Ogden | Apr 28 | Oct 12 | 167 d | 5a |
| Okoboji | Apr 28 | Oct 11 | 166 d | 5a |
| Olds | Apr 30 | Oct 10 | 163 d | 5b |
| Olin | Apr 27 | Oct 13 | 169 d | 5a |
| Ollie | Apr 24 | Oct 15 | 174 d | 5b |
| Onawa | May 1 | Oct 3 | 155 d | 5a |
| Onslow | Apr 28 | Oct 10 | 165 d | 5a |
| Oran | Apr 30 | Oct 9 | 162 d | 5a |
| Orange City | Apr 30 | Oct 3 | 156 d | 5a |
| Orchard | Apr 30 | Oct 9 | 162 d | 5a |
| Orient | Apr 25 | Oct 12 | 170 d | 5b |
| Osage | Apr 30 | Oct 9 | 162 d | 5a |
| Osceola | Apr 29 | Oct 9 | 163 d | 5b |
| Oskaloosa | Apr 21 | Oct 16 | 178 d | 5b |
| Ossian | May 1 | Oct 3 | 155 d | 5a |
| Otho | Apr 28 | Oct 8 | 163 d | 5a |
| Otley | Apr 20 | Oct 19 | 182 d | 5b |
| Oto | May 3 | Oct 1 | 151 d | 5a |
| Ottosen | Apr 27 | Oct 9 | 165 d | 5a |
| Ottumwa | Apr 23 | Oct 14 | 174 d | 5b |
| Oxford | Apr 28 | Oct 9 | 164 d | 5b |
| Oxford Junction | Apr 27 | Oct 10 | 166 d | 5a |
| Oyens | Apr 30 | Oct 4 | 157 d | 5a |