- Oakbrook Estates Mobile Home Park
- Oakcrest
- Oakglen
- Oakland Center
- Oakley
- Oakton
- Oakwood Estates
- Oakwood Heights
- Oasis Mobile Home Park
- Obed
- Ocoya
- Odgen
- Ogle
- Ogles
- Oil Center
- Oil Grove
- Oilfield
- Old Brownfield
- Old Colony Court
- Old Du Quoin
- Old Gilchrist
- Old Kane
- Old Kaskaskia
- Old Kent
- Old Marissa
- Old Niota
- Old Oaks
- Old Oaks Estates
- Old Pearl
- Old Princeton