- Oak Brook
- Oak Forest
- Oak Lawn
- Oak Park
- Oak Park Township
- Oakbrook Terrace
- Oakland
- Oakley
- Oakwood Township
- Oakwood
- Oakwood Hills
- Oblong
- Oblong Township
- Oconee Township
- Oconee
- Odell
- Odell Township
- Odin
- Odin Township
- Ogden Township
- Ogden
- Oglesby
- Ohio Township
- Okaw
- Old Town
- Olio
- Olive Branch
- Olive Branch Precinct
- Olney township
- Olney
- Olympia Fields
- Omega
- Onarga
- Onarga Township
- Oneco
- Opdyke
- Orange
- Orangeville
- Oreana
- Oregon
- Oregon-Nashua
- Orion
- Orland
- Orland Hills
- Orland Park
- Oswego
- Oswego Township
- Otego
- Ottawa
- Otter Creek
- Otto
- Owen
- Oxford
- Ozark