| Code | Name | Region |
|---|---|---|
| USOWB | Owensboro | KY |
| USTGH | Otsego | MI |
| USUY7 | Otsego | MN |
| USOTW | Ottawa | IL |
| USOTR | Ottawa | KS |
| USW8T | Ottawa | MN |
| USOTQ | Ottawa | OH |
| USOTL | Ottawa Lake | MI |
| USIXO | Otter River | MA |
| USO2T | Otterbein | IN |
| USOIM | Otterville | MO |
| USI8C | Otto | NC |
| USOTO | Otto | NM |
| USTOV | Ottoville | OH |
| USOSV | Ottsville | PA |
| USOTM | Ottumwa | IA |
| USOVZ | Otwell | AR |
| USOUR | Ouray | CO |
| US3VC | Outlook | WA |
| USKOZ | Ouzinkie | AK |
| USVFK | Oven Fork | KY |
| USO2S | Ossian | IN |
| USKZK | Ossineke | MI |
| USOSS | Ossining | NY |
| USN8H | Ossipee | NH |
| USOE2 | Osteen | FL |
| USOV3 | Osterville | MA |
| USOEH | Osthemo | MI |
| USORX | Ostrander | OH |
| USOST | Ostrica | LA |
| USOSG | Oswego | IL |
| USOSW | Oswego | NY |
| USOYM | Otay Mesa | CA |
| USOQG | Otego | NY |
| USXKF | Othello | WA |
| USO5T | Otis | CO |
| USTJI | Otis | KS |
| US8SI | Otis | MA |
| USOOW | Otis Orchards | WA |
| USOZV | Otisville | MI |
| USTSE | Otisville | NY |
| USOC9 | Osage City | KS |
| USOB4 | Osborne | KS |
| USOJB | Osburn | ID |
| USS4M | Oscar | MO |
| USOSA | Osceola | AR |
| USOCL | Osceola | IA |
| USM2S | Osceola | MO |
| USOEO | Osceola | WI |
| USOC3 | Osco | IL |