- Oak Brook
- Oak Forest
- Oak Lawn
- Oak Park
- Oakbrook Terrace
- Oakford
- Oakwood Hills
- Oblong
- Odin
- Oglesby
- Ohio
- Ohlman
- Okawville
- Old Mill Creek
- Old Ripley
- Old Shawneetown
- Olmsted
- Olympia Fields
- Onarga
- Oquawka
- Oreana
- Orland Hills
- Orland Park
- Owaneco
- Ocoya
- Oakbrook Estates Mobile Home Park
- Oak Ridge
- Orleans
- Oakwood Heights
- Oldenburg