City | 2024 Population |
|---|---|
| O'Fallon | 95,695 |
| Oak Grove | 8,784 |
| Oak Grove Village | 400 |
| Oak Ridge | 240 |
| Oakland | 1,360 |
| Oaks | 125 |
| Oakview | 357 |
| Oakville | 35,522 |
| Oakwood | 195 |
| Oakwood Park | 187 |
| Odessa | 5,603 |
| Old Appleton | 71 |
| Old Jamestown | 20,397 |
| Old Monroe | 274 |
| Olean | 121 |
| Olivette | 8,280 |
| Olympian Village | 709 |
| Oran | 1,196 |
| Oregon | 852 |
| Oronogo | 3,007 |
| Orrick | 730 |
| Osage | 1,654 |
| Osage Beach | 4,947 |
| Osborn | 381 |
| Osceola | 975 |
| Osgood | 22 |
| Otterville | 441 |
| Overland | 15,318 |
| Owensville | 2,805 |
| Ozark | 23,623 |
| Ozora | 391 |