- Old Reserve Hills
- Old River Island
- Old Salem Chautauqua
- Old Sawmill
- Old State Mobile Home Village
- Old Stonington
- Oldenburg
- Oldtown
- Olena
- Olga
- Olin
- Olive Branch
- Oliver
- Oliver
- Oliver Lane Subdivision
- Olivet
- Olson's Subdivision
- Omar
- Omega
- Oneco
- Ontario
- Ontarioville
- Opdyke
- Ophiem
- Orange
- Orange Prairie
- Orangemans Hall
- Oraville
- Orchard Subdivision
- Orchardville