| Code | Name | Region |
|---|---|---|
| USZE2 | Ohio City | OH |
| USXJV | Oconto Falls | WI |
| US5OT | Odebolt | IA |
| USODE | Odell | OR |
| USODM | Odem | TX |
| USODN | Odenton | MD |
| USQOD | Odenville | AL |
| USZGY | Odessa | DE |
| USODF | Odessa | FL |
| USO82 | Odessa | MN |
| USODG | Odessa | MO |
| USZOA | Odessa | NE |
| USODS | Odessa | NY |
| USOSD | Odessa | TX |
| USKQI | Odessa | WA |
| USQVJ | Odon | IN |
| USODO | O'donnell | TX |
| USOLW | Oelwein | IA |
| USOFA | O'fallon | IL |
| USOFL | O'fallon | MO |
| USOAB | Offshore Ambrose | |
| USOSP | Ocean Springs | MS |
| USOCX | Ocean View | DE |
| USOIW | Ocean View | NJ |
| USNTU | Oceana | VA |
| USOCI | Oceanic | AK |
| USVQC | Oceano | CA |
| USOCP | Oceanport | NJ |
| USOCN | Oceanside | CA |
| USXJX | Oceanside | NY |
| USOC5 | Oceanview | NJ |
| USONW | Oceanway | FL |
| USC7H | Ochelata | OK |
| USOCG | Ochlocknee | GA |
| USOC8 | Ochopee | FL |
| USOLA | Ocilla | GA |
| USOCM | Ocoee | FL |
| USOC7 | Ocoee | TN |
| USOZE | Ocoee, Polk | TN |
| USOCO | Oconomowoc | WI |
| USOCQ | Oconto | WI |
| USOAW | Oakwood, Buchanan | VA |
| USOWU | Oakwood, Oakland | MI |
| USOKG | Oakwood, Paulding | OH |
| USOXX | Oasis | CA |
| USOMX | Oatman | AZ |
| USXOR | Oberlin | OH |
| USOBE | Obetz | OH |
| USOBI | Obion | TN |
| USO6G | Oblong | IL |