| Code | Name | Region |
|---|---|---|
| USZNI | Onamia | MN |
| USOL2 | Oleum | CA |
| US3LY | Oley | PA |
| USKOY | Olga Bay | AK |
| USOXL | Olin | NC |
| USOIE | Olive | CA |
| USOQB | Olive Branch | IL |
| USOLV | Olive Branch | MS |
| USOHF | Olive Hill | KY |
| USOVH | Olivehurst | CA |
| USO7L | Olivet | MI |
| USZOT | Olivette | MO |
| USOIV | Olivia | MN |
| USOMG | Olmito | TX |
| USOZM | Olmsted | IL |
| USYOF | Olmsted Falls | OH |
| USOLY | Olney | IL |
| USOEY | Olney | MD |
| USOJN | Olney | MT |
| USONY | Olney | TX |
| USOFU | Olsburg | KS |
| USOFO | Old Fort | OH |
| USOFJ | Old Fort | TN |
| USO7G | Old Glory | TX |
| USOGW | Old Greenwich | CT |
| USOLH | Old Harbor | AK |
| USOHI | Old Hickory | AR |
| USXKA | Old Hickory | TN |
| USOLL | Old Lyme | CT |
| USOZI | Old Monroe | MO |
| USLDC | Old Ocean | TX |
| USOO2 | Old Orchard Beach | ME |
| USDKH | Old Rock Hill | GA |
| USOSY | Old Saybrook | CT |
| USOPN | Old Tappan | NJ |
| USOLD | Old Town | ME |
| USOW9 | Old Westbury | NY |
| USOH2 | Oldham | SD |
| USABA | Oldsmar | FL |
| USOXE | Oldtown | MD |
| USOLE | Olean | NY |
| USOKO | Oklahoma | TX |
| USOKC | Oklahoma City | OK |
| USOKJ | Oklawaha | FL |
| USOKM | Okmulgee | OK |
| USO2K | Okoboji | IA |
| USOKA | Okolona | MS |
| USOKZ | Okolona | OH |
| USO3L | Ola | AR |
| USZE3 | Olancha | CA |