City | 2024 Population |
|---|---|
| O'Fallon | 31,977 |
| Oak Brook | 7,901 |
| Oak Forest | 25,550 |
| Oak Grove | 462 |
| Oak Lawn | 54,403 |
| Oak Park | 50,710 |
| Oak Run | 759 |
| Oakbrook Terrace | 2,646 |
| Oakdale | 197 |
| Oakford | 227 |
| Oakland | 720 |
| Oakley | 931 |
| Oakwood | 1,267 |
| Oakwood Hills | 2,054 |
| Oblong | 1,351 |
| Oconee | 138 |
| Odell | 980 |
| Odin | 897 |
| Ogden | 711 |
| Oglesby | 3,577 |
| Ohio | 457 |
| Ohio Grove | 268 |
| Ohlman | 109 |
| Okaw | 872 |
| Okawville | 1,347 |
| Old Mill Creek | 159 |
| Old Ripley | 82 |
| Old Shawneetown | 114 |
| Old Town | 3,420 |
| Olio | 4,637 |
| Olive | 1,660 |
| Olive Branch | 546 |
| Olmsted | 266 |
| Olney | 8,288 |
| Olympia Fields | 4,426 |
| Omaha | 198 |
| Omega | 428 |
| Omphghent | 2,364 |
| Onarga | 1,265 |
| Oneco | 1,235 |
| Oneida | 662 |
| Ontario | 869 |
| Opdyke | 253 |
| Ophiem | 184 |
| Ophir | 425 |
| Oquawka | 1,052 |
| Ora | 499 |
| Oran | 301 |
| Orange | 252 |
| Orangeville | 738 |
| Orchard | 604 |
| Oreana | 845 |
| Oregon | 3,543 |
| Orel | 1,318 |
| Orient | 317 |
| Orion | 1,699 |
| Orland | 93,640 |
| Orland Hills | 6,419 |
| Orland Park | 56,455 |
| Orvil | 937 |
| Osage | 243 |
| Osceola | 903 |
| Osco | 128 |
| Oskaloosa | 330 |
| Oswego | 36,981 |
| Otego | 1,403 |
| Ottawa | 18,488 |
| Otter Creek | 846 |
| Otterville | 85 |
| Otto | 2,040 |
| Owaneco | 205 |
| Owego | 297 |
| Owen | 3,532 |
| Oxford | 1,107 |
| Oxville | 21 |