Pennsylvania frost dates: O
| City | Last frost | First frost | Season | Zone |
|---|---|---|---|---|
| Oak Ridge | May 10 | Oct 12 | 155 d | 6a |
| Oakdale | Apr 23 | Oct 23 | 183 d | 6b |
| Oakland Mills | Apr 21 | Oct 24 | 186 d | 6b |
| Oakmont | Apr 16 | Nov 1 | 199 d | 6b |
| Oaks | Apr 18 | Oct 27 | 192 d | 7a |
| Ohiopyle | May 19 | Oct 1 | 135 d | 6a |
| Oil City | May 2 | Oct 23 | 174 d | 6a |
| Olanta | May 4 | Oct 13 | 162 d | 6a |
| Old Forge | Apr 23 | Oct 20 | 180 d | 7a |
| Old Zionsville | Apr 26 | Oct 14 | 171 d | 7a |
| Oley | Apr 10 | Oct 31 | 204 d | 7a |
| Oliveburg | May 10 | Oct 12 | 155 d | 6a |
| Oliver | Apr 30 | Oct 18 | 171 d | 6b |
| Olyphant | Apr 23 | Oct 20 | 180 d | 6a |
| Oneida | Apr 30 | Oct 22 | 175 d | 6b |
| Ono | Apr 20 | Oct 25 | 188 d | 7a |
| Orangeville | May 4 | Oct 11 | 160 d | 6b |
| Orbisonia | Apr 19 | Oct 28 | 192 d | 7a |
| Orefield | Apr 20 | Oct 24 | 187 d | 7a |
| Oreland | Apr 11 | Nov 1 | 204 d | 7b |
| Orrstown | Apr 19 | Oct 24 | 188 d | 7a |
| Orrtanna | Apr 28 | Oct 17 | 172 d | 7a |
| Orson | May 14 | Oct 1 | 140 d | 5b |
| Orviston | May 21 | Sep 27 | 129 d | 6b |
| Orwigsburg | Apr 16 | Oct 25 | 192 d | 7a |
| Osceola | May 14 | Oct 5 | 144 d | 5b |
| Osceola Mills | May 13 | Oct 1 | 141 d | 6a |
| Osterburg | Apr 29 | Oct 13 | 167 d | 6b |
| Ottsville | Apr 29 | Oct 16 | 170 d | 6b |
| Oxford | Apr 26 | Oct 22 | 179 d | 7a |