- Oraville
- O Fallon
- Oak Brook
- Oak Forest
- Oak Grove
- Oak Lawn
- Oak Park
- Oak Park Township
- Oakford
- Oakland
- Oblong Township
- Oblong
- Odell Township
- Odell
- Odin
- Odin Township
- O'fallon
- Oglesby
- Ohlman
- Olney township
- Olney
- Olympia Fields
- Oneida
- Opdyke
- Ophiem
- Oquawka Township
- Oquawka
- Orangeville
- Oreana
- Oregon
- Orient
- Orion
- Orland Park
- Osco
- Oswego
- Oswego Township
- Ottawa
- Owaneco
- Ozark