| Code | Name | Region |
|---|---|---|
| US4CA | Orinda | CA |
| USORG | Orange | TX |
| USOGE | Orange | VA |
| USOGX | Orange | VT |
| USOBC | Orange Beach | AL |
| USOGC | Orange City | FL |
| USOC2 | Orange City | IA |
| USOCC | Orange Cove | CA |
| USORP | Orange Park | FL |
| USOBG | Orangeburg | NY |
| USOGB | Orangeburg | SC |
| USOEV | Orangevale | CA |
| USOAG | Orangeville | IL |
| USOV2 | Orangeville | PA |
| USOGV | Orangeville | UT |
| USOSZ | Orca | AK |
| USXJY | Orcas | WA |
| USOR5 | Orchard | IA |
| USOCD | Orchard | NE |
| USOQP | Orchard Beach | MD |
| USOHK | Orchard Lake | MI |
| USONO | Ontario | OR |
| USW7I | Ontario | WI |
| USONG | Ontonagon | MI |
| USOO3 | Oologah | OK |
| USZOW | Ooltewah | TN |
| USOSU | Oostburg | WI |
| USOPK | Opa Locka | FL |
| USOP8 | Opal | WY |
| USOPI | Opelika | AL |
| 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 |