| Code | Name | Region |
|---|---|---|
| USZOG | Orogrande | NM |
| USOPY | Orchard Park | NY |
| USOCJ | Orchards | WA |
| US8MN | Orcutt | CA |
| USOJR | Ord | NE |
| USORY | Ordbend | CA |
| USODY | Ordinary | VA |
| US9OC | Ore City | TX |
| USXKD | Orefield | PA |
| USOGI | Oregon | IL |
| USOEG | Oregon | OH |
| USOR9 | Oregon | WI |
| USORC | Oregon City | OR |
| USOJG | Oregon House | CA |
| USODJ | Oreland | PA |
| USOEU | Orem | UT |
| USORU | Orestes | IN |
| USOF2 | Orford | NH |
| USOQE | Orick | CA |
| USO8R | Orient | ME |
| 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 |