| Code | Name | Region |
|---|---|---|
| USOSC | Oscoda | MI |
| USYZD | Osgood | IN |
| USOKS | Oshkosh | NE |
| USOSH | Oshkosh | WI |
| USOOA | Oskaloosa | IA |
| USOS3 | Oskaloosa | KS |
| USOSL | Oslo | MN |
| USODQ | Osmond | NE |
| USOS2 | Osseo | MI |
| USOXS | Osseo | MN |
| USOZZ | Osseo | WI |
| USYDO | Orondo | WA |
| US2OR | Orono | ME |
| USR3N | Orono | MN |
| USOO8 | Oronoco | MN |
| USORO | Orosi | CA |
| USOVE | Oroville | CA |
| USOVW | Oroville | WA |
| USIHK | Orrick | MO |
| USOR7 | Orrington | ME |
| 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 |