- Old Oaks
- Old Oaks Estates
- Old Pearl
- Old Princeton
- Old Reserve Hills
- Old Ripley
- Old River Island
- Old Salem Chautauqua
- Old Sawmill
- Old Shawneetown
- Old State Mobile Home Village
- Old Stonington
- Oldenburg
- Oldtown
- Olena
- Olga
- Olin
- Olive Branch
- Oliver
- Oliver
- Oliver Lane Subdivision
- Olivet
- Olmsted
- Olney
- Olson's Subdivision
- Olympia Fields
- Omaha
- Omar
- Omega
- Onarga