City | 2024 Population |
|---|---|
| O'Fallon | 94,891 |
| Oak Grove | 9,142 |
| Oak Grove Village | 397 |
| Oak Ridge | 238 |
| Oakland | 1,363 |
| Oaks | 126 |
| Oakview | 361 |
| Oakville | 35,522 |
| Oakwood | 195 |
| Oakwood Park | 184 |
| Odessa | 5,648 |
| Old Appleton | 71 |
| Old Jamestown | 20,397 |
| Old Monroe | 275 |
| Olean | 119 |
| Olivette | 8,954 |
| Olympian Village | 713 |
| Oran | 1,206 |
| Oregon | 837 |
| Oronogo | 3,004 |
| Orrick | 737 |
| Osage | 1,654 |
| Osage Beach | 4,909 |
| Osborn | 375 |
| Osceola | 983 |
| Osgood | 23 |
| Otterville | 445 |
| Overland | 15,356 |
| Owensville | 2,792 |
| Ozark | 23,408 |
| Ozora | 391 |