| Code | Name | Region |
|---|---|---|
| USOWN | Orrstown | PA |
| USOTP | Orrtanna | PA |
| USORR | Orrville | OH |
| USXKE | Orting | WA |
| USOOI | Ortonville | MI |
| USOEQ | Orwell | OH |
| USORW | Orwell | VT |
| USOWP | Orwigsburg | PA |
| USOAE | Osage | IA |
| USOG8 | Osage | WY |
| USOSB | Osage Beach | MO |
| USOR2 | Orion | IL |
| USO6N | Orion | MI |
| USOKY | Oriskany | NY |
| USOFY | Oriskany Falls | NY |
| USOR3 | Orla | TX |
| USORN | Orland | CA |
| USOLJ | Orland | IN |
| USODX | Orland | ME |
| USXKC | Orland Park | IL |
| USORL | Orlando | FL |
| USOFZ | Orleans | CA |
| USORZ | Orleans | IN |
| USOLN | Orleans | MA |
| USORJ | Orleans | VT |
| USQOL | Orlinda | TN |
| USODB | Ormond Beach | FL |
| USOOQ | Oro Grande | CA |
| USOVU | Oro Valley | AZ |
| USOFG | Orofino | ID |
| 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 |