| Code | Name | Region |
|---|---|---|
| USOXT | Olanta | PA |
| USFIZ | Olar | SC |
| USOZT | Olathe | CO |
| USOLB | Olathe | KS |
| USOHA | Olathia | MO |
| USOPG | Old Bethpage | NY |
| USODR | Old Bridge | NJ |
| USDXX | Old Brookville | NY |
| USOC6 | Old Chatham | NY |
| USXJZ | Old Forge | PA |
| USOFT | Old Fort | NC |
| USOHY | Ohiopyle | PA |
| USOHV | Ohioview | PA |
| USOYX | Ohkay Owingeh | NM |
| USOTY | Oil City | LA |
| USOIL | Oil City | PA |
| USODC | Oildale | CA |
| USIUN | Oilton | OK |
| USITB | Oilton | TX |
| USOVB | Oilville | VA |
| 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 |