| Name | What | Region | Country | Lat | Long | Elev ft. | Pop est |
|---|---|---|---|---|---|---|---|
| O'Fallon | city | Illinois | United States | 38.59 | -89.91 | 830 | |
| Oak | city | Illinois | United States | 37.58 | -88.52 | 830 | |
| Oak Brook | city | Illinois | United States | 41.83 | -87.93 | 830 | |
| Oak Forest | city | Illinois | United States | 41.60 | -87.74 | 830 | |
| Oak Grove | city | Illinois | United States | 42.30 | -87.92 | 830 | |
| Oak Grove | city | Illinois | United States | 41.41 | -90.57 | 830 | |
| Oak Grove | city | Illinois | United States | 40.02 | -90.41 | 830 | |
| Oak Hill | city | Illinois | United States | 40.79 | -89.87 | 830 | |
| Oak Hills | city | Illinois | United States | 38.64 | -89.98 | 830 | |
| Oak Lane | city | Illinois | United States | 41.48 | -88.20 | 830 | |
| Oak Lawn | city | Illinois | United States | 41.71 | -87.76 | 830 | |
| Oak Lawn Mobile Home Park | city | Illinois | United States | 42.34 | -88.85 | 872 | |
| Oak Openings | city | Illinois | United States | 41.93 | -88.46 | 948 | |
| Oak Park | city | Illinois | United States | 41.89 | -87.78 | 830 | |
| Oak Point | city | Illinois | United States | 39.21 | -87.98 | 830 | |
| Oak Ridge | city | Illinois | United States | 42.16 | -88.09 | 859 | |
| Oak Ridge | city | Illinois | United States | 40.81 | -89.44 | 830 | |
| Oak Ridge North | city | Illinois | United States | 42.16 | -88.09 | 862 | |
| Oak Run | city | Illinois | United States | 40.96 | -90.15 | 830 | |
| Oak Valley | city | Illinois | United States | 41.60 | -87.93 | 830 | |
| Oakbrook Estates Mobile Home Park | city | Illinois | United States | 42.22 | -88.28 | 872 | |
| Oakbrook Terrace | city | Illinois | United States | 41.85 | -87.96 | 830 | |
| Oakcrest | city | Illinois | United States | 39.38 | -87.71 | 830 | |
| Oakdale | city | Illinois | United States | 38.26 | -89.50 | 830 | |
| Oakenwald | city | Illinois | United States | 41.83 | -87.61 | 830 | |
| Oakford | city | Illinois | United States | 40.10 | -89.96 | 830 | |
| Oakglen | city | Illinois | United States | 41.56 | -87.56 | 830 | |
| Oakland | city | Illinois | United States | 39.65 | -88.03 | 830 | |
| Oakland | city | Illinois | United States | 41.82 | -87.60 | 830 | |
| Oakland Center | city | Illinois | United States | 40.23 | -90.51 | 830 | |
| Oakley | city | Illinois | United States | 39.88 | -88.81 | 830 | |
| Oakton | city | Illinois | United States | 42.03 | -87.67 | 830 | |
| Oakwood | city | Illinois | United States | 40.12 | -87.78 | 830 | |
| Oakwood Estates | city | Illinois | United States | 41.52 | -88.17 | 830 | |
| Oakwood Heights | city | Illinois | United States | 38.88 | -90.07 | 830 | |
| Oakwood Hills | city | Illinois | United States | 42.25 | -88.24 | 830 | |
| Oasis Mobile Home Park | city | Illinois | United States | 42.01 | -87.94 | 830 | |
| Oasis Trailer Park | city | Illinois | United States | 40.50 | -89.01 | 830 | |
| Obed | city | Illinois | United States | 39.57 | -88.90 | 830 | |
| Oblong | city | Illinois | United States | 39.00 | -87.91 | 830 | |
| Oconee | city | Illinois | United States | 39.29 | -89.11 | 830 | |
| Ocoya | city | Illinois | United States | 40.81 | -88.68 | 830 | |
| Odell | city | Illinois | United States | 41.00 | -88.53 | 830 | |
| Odgen | city | Illinois | United States | 38.45 | -87.83 | 830 | |
| Odin | city | Illinois | United States | 38.62 | -89.05 | 830 | |
| Ogden | city | Illinois | United States | 40.11 | -87.96 | 830 | |
| Ogle | city | Illinois | United States | 41.11 | -90.88 | 830 | |
| Ogles | city | Illinois | United States | 38.55 | -90.05 | 830 | |
| Oglesby | city | Illinois | United States | 41.30 | -89.06 | 830 | |
| Ohio | city | Illinois | United States | 41.56 | -89.46 | 892 | |
| Ohlman | city | Illinois | United States | 39.35 | -89.22 | 830 | |
| Oil Center | city | Illinois | United States | 38.97 | -87.83 | 830 | |
| Oil Grove | city | Illinois | United States | 38.86 | -87.64 | 830 | |
| Oilfield | city | Illinois | United States | 39.39 | -87.99 | 830 | |
| Okawville | city | Illinois | United States | 38.43 | -89.55 | 830 | |
| Old Brownfield | city | Illinois | United States | 37.36 | -88.60 | 830 | |
| Old Colony Court | city | Illinois | United States | 42.13 | -87.84 | 830 | |
| Old Du Quoin | city | Illinois | United States | 37.98 | -89.18 | 400 | |
| Old Gilchrist | city | Illinois | United States | 41.23 | -90.63 | 830 | |
| Old Kane | city | Illinois | United States | 39.19 | -90.37 | 830 | |
| Old Kaskaskia | city | Illinois | United States | 37.95 | -89.92 | 367 | |
| Old Kent | city | Illinois | United States | 41.52 | -88.16 | 830 | |
| Old Marissa | city | Illinois | United States | 38.26 | -89.75 | 830 | |
| Old Mill Creek | city | Illinois | United States | 42.42 | -87.97 | 830 | |
| Old Mill Grove | city | Illinois | United States | 42.20 | -88.08 | 843 | |
| Old Niota | city | Illinois | United States | 40.61 | -91.30 | 830 | |
| Old Oaks | city | Illinois | United States | 41.57 | -88.18 | 830 | |
| Old Oaks Estates | city | Illinois | United States | 41.64 | -87.94 | 830 | |
| Old Pearl | city | Illinois | United States | 39.44 | -90.65 | 830 | |
| Old Princeton | city | Illinois | United States | 39.88 | -90.16 | 830 | |
| Old Reserve Hills | city | Illinois | United States | 41.65 | -88.35 | 830 | |
| Old Ripley | city | Illinois | United States | 38.90 | -89.57 | 830 | |
| Old Salem Chautauqua | city | Illinois | United States | 39.99 | -89.84 | 830 | |
| Old Sawmill | city | Illinois | United States | 41.72 | -88.15 | 830 | |
| Old Shawneetown | city | Illinois | United States | 37.70 | -88.14 | 341 | |
| Old State Mobile Home Village | city | Illinois | United States | 39.45 | -88.37 | 830 | |
| Old Stonington | city | Illinois | United States | 39.61 | -89.15 | 830 | |
| Oldenburg | city | Illinois | United States | 38.80 | -90.10 | 830 | |
| Oldtown | city | Illinois | United States | 37.60 | -88.69 | 830 | |
| Olena | city | Illinois | United States | 40.79 | -90.94 | 830 | |
| Olga | city | Illinois | United States | 37.99 | -88.58 | 413 | |
| Olin | city | Illinois | United States | 40.75 | -89.77 | 830 | |
| Olive Branch | city | Illinois | United States | 37.17 | -89.35 | 331 | |
| Oliver | city | Illinois | United States | 41.80 | -90.07 | 830 | |
| Oliver | city | Illinois | United States | 39.48 | -87.68 | 830 | |
| Oliver Lane Subdivision | city | Illinois | United States | 42.11 | -87.86 | 830 | |
| Olivet | city | Illinois | United States | 39.94 | -87.64 | 830 | |
| Olmsted | city | Illinois | United States | 37.18 | -89.09 | 367 | |
| Olney | city | Illinois | United States | 38.73 | -88.09 | 830 | |
| Olsen-Hansen Row Houses | city | Illinois | United States | 41.91 | -87.63 | 830 | |
| Olson's Subdivision | city | Illinois | United States | 42.14 | -87.85 | 830 | |
| Olympia Fields | city | Illinois | United States | 41.51 | -87.67 | 830 | |
| Omaha | city | Illinois | United States | 37.89 | -88.30 | 360 | |
| Omar | city | Illinois | United States | 37.50 | -88.97 | 830 | |
| Omega | city | Illinois | United States | 38.70 | -88.78 | 830 | |
| Onarga | city | Illinois | United States | 40.72 | -88.01 | 830 | |
| Oneco | city | Illinois | United States | 42.49 | -89.66 | 849 | |
| Oneida | city | Illinois | United States | 41.07 | -90.23 | 830 | |
| Ontario | city | Illinois | United States | 41.08 | -90.31 | 830 | |
| Ontarioville | city | Illinois | United States | 41.99 | -88.15 | 830 | |
| Opdyke | city | Illinois | United States | 38.26 | -88.79 | 830 | |
| Ophiem | city | Illinois | United States | 41.25 | -90.39 | 830 | |
| Oquawka | city | Illinois | United States | 40.93 | -90.95 | 830 | |
| Orange | city | Illinois | United States | 39.21 | -87.82 | 830 | |
| Orange Prairie | city | Illinois | United States | 40.78 | -89.67 | 830 | |
| Orangemans Hall | city | Illinois | United States | 41.31 | -90.61 | 830 | |
| Orangeville | city | Illinois | United States | 42.47 | -89.64 | 849 | |
| Oraville | city | Illinois | United States | 37.87 | -89.38 | 400 | |
| Orchard Subdivision | city | Illinois | United States | 42.12 | -87.82 | 830 | |
| Orchardville | city | Illinois | United States | 38.51 | -88.65 | 830 | |
| Oreana | city | Illinois | United States | 39.94 | -88.87 | 830 | |
| Oregon | city | Illinois | United States | 42.01 | -89.33 | 830 | |
| Orient | city | Illinois | United States | 37.92 | -88.98 | 400 | |
| Originally named Warren, IL in 1840. | city | Illinois | United States | 40.79 | -90.99 | 830 | |
| Orio | city | Illinois | United States | 38.55 | -87.78 | 830 | |
| Orion | city | Illinois | United States | 41.35 | -90.38 | 830 | |
| Orland Hills | city | Illinois | United States | 41.59 | -87.84 | 830 | |
| Orland Park | city | Illinois | United States | 41.63 | -87.85 | 830 | |
| Orleans | city | Illinois | United States | 39.72 | -90.08 | 830 | |
| Ormonde | city | Illinois | United States | 40.86 | -90.61 | 830 | |
| Osbernville | city | Illinois | United States | 39.76 | -89.18 | 830 | |
| Osborn | city | Illinois | United States | 41.52 | -90.27 | 830 | |
| Osceola | city | Illinois | United States | 41.21 | -89.78 | 830 | |
| Osco | city | Illinois | United States | 41.35 | -90.28 | 830 | |
| Oskaloosa | city | Illinois | United States | 38.76 | -88.65 | 830 | |
| Osman | city | Illinois | United States | 40.30 | -88.47 | 830 | |
| Ospur | city | Illinois | United States | 40.09 | -88.96 | 830 | |
| Oswego | city | Illinois | United States | 41.68 | -88.35 | 830 | |
| Oswego Village Square | city | Illinois | United States | 41.69 | -88.37 | 830 | |
| Ottawa | city | Illinois | United States | 41.35 | -88.84 | 830 | |
| Otter (historical) | city | Illinois | United States | 41.18 | -88.78 | 830 | |
| Otterville | city | Illinois | United States | 39.05 | -90.40 | 830 | |
| Otto | city | Illinois | United States | 41.05 | -87.89 | 830 | |
| Otto (historical) | city | Illinois | United States | 40.30 | -90.21 | 830 | |
| Ottville | city | Illinois | United States | 41.35 | -89.26 | 830 | |
| Oury Point | city | Illinois | United States | 42.13 | -87.81 | 830 | |
| Owaneco | city | Illinois | United States | 39.48 | -89.19 | 830 | |
| Owen Center | city | Illinois | United States | 42.37 | -89.12 | 830 | |
| Oxford (historical) | city | Illinois | United States | 41.17 | -90.43 | 830 | |
| Oxville | city | Illinois | United States | 39.70 | -90.56 | 830 | |
| Ozark | city | Illinois | United States | 37.54 | -88.76 | 830 |