Illinois cities which start with the letter O.
| Place | Place Type | 2010 Population |
|---|---|---|
| O Fallon | ||
| Oak Brk Mall | ||
| Oak Brook | village | 7796 |
| Oak Brook Mall | ||
| Oak Forest | city | 27962 |
| Oak Lawn | village | 56690 |
| Oak Park | village | 51878 |
| Oak Ridge | ||
| Oak Run | ||
| Oakbrook Ter | ||
| Oakbrook Terrace | city | 2134 |
| Oakdale | village | 221 |
| Oakford | village | 286 |
| Oakland | township | 135 |
| Oakley | township | 1082 |
| Oakwood | village | 1595 |
| Oakwood Hills | village | 2030 |
| Oblong | village | 1466 |
| Oconee | village | 180 |
| Ocoya | ||
| Odell | village | 1046 |
| Odgen | ||
| Odin | village | 1076 |
| Ogden | village | 810 |
| Ogden Park | ||
| Oglesby | city | 3791 |
| Ohio | village | 513 |
| Ohio Grove | township | 279 |
| Ohlman | village | 135 |
| Okawville | village | 1434 |
| Old Berlin | ||
| Old Camp | ||
| Old Gilchrist | ||
| Old Kane | ||
| Old Mill Creek | village | 16 |
| Old Ripley | village | 108 |
| Old Shawneetown | village | 193 |
| Old Stonington | ||
| Oldtown | ||
| Olena | ||
| Olive Branch | ||
| Oliver | ||
| Olmsted | village | 333 |
| Olney | city | 9115 |
| Olympia Fields | village | 4471 |
| Olympia Flds | ||
| Omaha | village | 266 |
| Omega | township | 501 |
| Onarga | village | 1368 |
| Oneco | township | 1331 |
| Oneida | city | 700 |
| Ontario Street | ||
| Ontarioville | ||
| Opdyke | ||
| Opheim | ||
| Ophiem | ||
| Oquawka | village | 1371 |
| Orangeville | village | 789 |
| Oraville | ||
| Orchard Heights | ||
| Orchard Mines | ||
| Orchardville | ||
| Oreana | village | 875 |
| Oregon | city | 3721 |
| Orel | township | 1420 |
| Orient | city | 358 |
| Orion | village | 1861 |
| Orland Hills | village | 7149 |
| Orland Park | village | 184 |
| Ormonde | ||
| Osceola | township | 1014 |
| Osco | township | 459 |
| Oskaloosa | township | 322 |
| Ospur | ||
| Ossami Lake | ||
| Oswego | village | 27687 |
| Otego | township | 1511 |
| Otis Grant Collins | ||
| Ottawa | city | 0 |
| Otter Creek | township | 2970 |
| Otterville | town | 126 |
| Otto Mall | ||
| Ottoville | ||
| Owaneco | village | 239 |
| Owego | township | 328 |
| Ozark |
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