| Code | Name | Region |
|---|---|---|
| USONF | Oneonta | AL |
| USONH | Oneonta | NY |
| USOID | Onida | SD |
| USONN | Onion Bay | AK |
| USONP | Ono | PA |
| USOGQ | Onondaga | NY |
| USONS | Onset | MA |
| USOTD | Onsted | MI |
| USONT | Ontario | CA |
| USOTF | Ontario | NY |
| USOT5 | Ontario | OH |
| USQOT | Olton | TX |
| USOEJ | Olustee | FL |
| USOLM | Olympia | WA |
| USOAN | Olympia Fields | IL |
| USZOY | Olympic Valley | CA |
| USQOA | Olyphant | PA |
| USOH8 | Omaha | AR |
| USOM3 | Omaha | IL |
| USOMA | Omaha | NE |
| USOMK | Omak | WA |
| US9OM | Omega | GA |
| USO5M | Omena | MI |
| USOM2 | Omro | WI |
| USON3 | Ona | FL |
| USOA7 | Ona | WV |
| USKGA | Onaga | KS |
| USO9K | Onalaska | TX |
| USOAA | Onalaska | WA |
| USZOK | Onalaska | WI |
| USZNI | Onamia | MN |
| USOL2 | Oleum | CA |
| US3LY | Oley | PA |
| USKOY | Olga Bay | AK |
| USOXL | Olin | NC |
| USOIE | Olive | CA |
| USOQB | Olive Branch | IL |
| USOLV | Olive Branch | MS |
| USOHF | Olive Hill | KY |
| USOVH | Olivehurst | CA |
| USO7L | Olivet | MI |
| USZOT | Olivette | MO |
| USOIV | Olivia | MN |
| USOMG | Olmito | TX |
| USOZM | Olmsted | IL |
| USYOF | Olmsted Falls | OH |
| USOLY | Olney | IL |
| USOEY | Olney | MD |
| USOJN | Olney | MT |
| USONY | Olney | TX |