Illinois frost dates: O
| City | Last frost | First frost | Season | Zone |
|---|---|---|---|---|
| O Fallon | Apr 12 | Oct 20 | 191 d | 7a |
| Oak Brook | Apr 28 | Oct 16 | 171 d | 6a |
| Oak Forest | Apr 23 | Oct 20 | 180 d | 6a |
| Oak Lawn | Apr 11 | Oct 31 | 203 d | 6a |
| Oak Park | Apr 11 | Oct 31 | 203 d | 6a |
| Oakdale | Apr 12 | Oct 22 | 193 d | 6b |
| Oakford | Apr 13 | Oct 21 | 191 d | 6a |
| Oakland | Apr 15 | Oct 23 | 191 d | 6a |
| Oakwood | Apr 21 | Oct 20 | 182 d | 6a |
| Oblong | Apr 13 | Oct 22 | 192 d | 6b |
| Oconee | Apr 17 | Oct 23 | 189 d | 6b |
| Odell | Apr 27 | Oct 15 | 171 d | 6a |
| Odin | Apr 9 | Oct 26 | 200 d | 6b |
| Ogden | Apr 20 | Oct 21 | 184 d | 6a |
| Oglesby | Apr 24 | Oct 18 | 177 d | 5b |
| Ohio | Apr 24 | Oct 14 | 173 d | 5b |
| Ohlman | Apr 17 | Oct 23 | 189 d | 6b |
| Okawville | Apr 7 | Oct 26 | 202 d | 6b |
| Olive Branch | Apr 5 | Oct 25 | 203 d | 7a |
| Olmsted | Apr 3 | Nov 5 | 216 d | 7a |
| Olney | Apr 15 | Oct 23 | 191 d | 6b |
| Olympia Fields | Apr 23 | Oct 20 | 180 d | 6a |
| Omaha | Apr 15 | Oct 19 | 187 d | 7a |
| Onarga | Apr 25 | Oct 20 | 178 d | 5b |
| Oneida | Apr 28 | Oct 12 | 167 d | 5b |
| Opdyke | Apr 11 | Oct 22 | 194 d | 6b |
| Ophiem | Apr 28 | Oct 12 | 167 d | 5b |
| Oquawka | Apr 8 | Oct 29 | 204 d | 5b |
| Orangeville | Apr 27 | Oct 14 | 170 d | 5a |
| Oraville | Apr 9 | Oct 22 | 196 d | 7a |
| Oreana | Apr 16 | Oct 22 | 189 d | 6a |
| Oregon | Apr 29 | Oct 7 | 161 d | 5b |
| Orient | Apr 4 | Oct 27 | 206 d | 7a |
| Orion | Apr 24 | Oct 18 | 177 d | 5b |
| Orland Park | Apr 19 | Oct 22 | 186 d | 6a |
| Osco | Apr 20 | Oct 20 | 183 d | 5b |
| Oswego | Apr 19 | Oct 23 | 187 d | 5b |
| Ottawa | Apr 20 | Oct 22 | 185 d | 5b |
| Owaneco | Apr 17 | Oct 23 | 189 d | 6a |
| Ozark | Apr 14 | Oct 21 | 190 d | 7a |