| Code | Name | Region |
|---|---|---|
| USOJA | Ojai | CA |
| USOHP | Okahumpka | FL |
| USO4G | Okanogan | WA |
| USZHE | Okarche | OK |
| USQXW | Okawville | IL |
| USOZU | Okeana | OH |
| USOKB | Okeechobee | FL |
| USOLT | Okeelanta | FL |
| USOKN | Okeene | OK |
| USOMJ | Okemah | OK |
| USOKX | Okemos | MI |
| USOCT | Offshore Corpus Christi | |
| USOF8 | Offutt | NE |
| USOGA | Ogallala | NE |
| USOGO | Ogden | IA |
| USOGZ | Ogden | IL |
| USA2X | Ogden | NC |
| USOGD | Ogden | UT |
| USOGU | Ogdensburg | NJ |
| USOGS | Ogdensburg | NY |
| USOG2 | Ogdensburg | WI |
| USOGF | Ogilvie | MN |
| USOGY | Oglesby | IL |
| USOHQ | Oglethorpe | GA |
| USYOQ | Ogunquit | ME |
| USORD | O'hare Apt/chicago | IL |
| USOHH | Ohatchee | AL |
| USOH3 | Ohio | IL |
| USOHB | Ohio | PA |
| USQ32 | Ohio City | CO |
| USZE2 | Ohio City | OH |
| USXJV | Oconto Falls | WI |
| US5OT | Odebolt | IA |
| USODE | Odell | OR |
| USODM | Odem | TX |
| USODN | Odenton | MD |
| USQOD | Odenville | AL |
| USZGY | Odessa | DE |
| USODF | Odessa | FL |
| USO82 | Odessa | MN |
| USODG | Odessa | MO |
| USZOA | Odessa | NE |
| USODS | Odessa | NY |
| USOSD | Odessa | TX |
| USKQI | Odessa | WA |
| USQVJ | Odon | IN |
| USODO | O'donnell | TX |
| USOLW | Oelwein | IA |
| USOFA | O'fallon | IL |
| USOFL | O'fallon | MO |