| Code | Name | Region |
|---|---|---|
| USOPL | Opelousas | LA |
| USOPH | Opheim | MT |
| USOPA | Opp | AL |
| USORA | Oradell | NJ |
| USO2R | Oran | MO |
| USJOR | Orange | CA |
| USORB | Orange | CT |
| USOGJ | Orange | FL |
| USOAF | Orange | MA |
| USJNJ | Orange | NJ |
| US2OV | Orange | OH |
| USON2 | Onarga | IL |
| USIA3 | Onawa | IA |
| USOW2 | Onaway | MI |
| USONE | Oneco | FL |
| USOND | Oneida | NY |
| USOYI | Oneida | TN |
| USONI | Oneida | WI |
| USONL | O'neill | NE |
| USONK | Onekama | MI |
| 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 |