- O'Fallon
- Oak
- Oak Brook
- Oak Forest
- Oak Grove
- Oak Grove
- Oak Hill
- Oak Hills
- Oak Lane
- Oak Lawn
- Oak Lawn Mobile Home Park
- Oak Openings
- Oak Park
- Oak Park Township
- Oak Point
- Oak Ridge
- Oak Run
- Oak Valley
- Oakbrook Estates Mobile Home Park
- Oakbrook Terrace
- Oakcrest
- Oakdale
- Oakford
- Oakglen
- Oakland
- Oakland Center
- Oakley
- Oakton
- Oakwood
- Oakwood Estates