| Code | Name | Region |
|---|---|---|
| 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 |
| 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 |