Towns
| State | City/Town | Town Title |
|---|---|---|
| Illinois | Oak Brook | Oak Brook, IL |
| Illinois | Oak Forest | Oak Forest, IL |
| Illinois | Oak Lawn | Oak Lawn, IL |
| Illinois | Oak Park | Oak Park, IL |
| Illinois | Oakdale | Oakdale, IL |
| Illinois | Oakford | Oakford, IL |
| Illinois | Oakland | Oakland, IL |
| Illinois | Oakwood | Oakwood, IL |
| Illinois | Oblong | Oblong, IL |
| Illinois | Oconee | Oconee, IL |
| Illinois | Odell | Odell, IL |
| Illinois | Odin | Odin, IL |
| Illinois | Ogden | Ogden, IL |
| Illinois | Oglesby | Oglesby, IL |
| Illinois | Ohio | Ohio, IL |
| Illinois | Ohlman | Ohlman, IL |
| Illinois | Okawville | Okawville, IL |
| Illinois | Olive Branch | Olive Branch, IL |
| Illinois | Olmsted | Olmsted, IL |
| Illinois | Olney | Olney, IL |
| Illinois | Olympia Fields | Olympia Fields, IL |
| Illinois | Omaha | Omaha, IL |
| Illinois | Onarga | Onarga, IL |
| Illinois | Oneida | Oneida, IL |
| Illinois | Opdyke | Opdyke, IL |