- Ogilville
- Old Adamsboro
- Old Bargersville
- Old Bath
- Old Halfway
- Old Hill
- Old Milan
- Old Pekin
- Old Saint Louis
- Old Tip Town
- Old Town
- Oldenburg
- Oldtown
- Olean
- Olin
- Olio
- Olive
- Oliver
- Omega
- Ontario
- Onward
- Oolitic
- Ora
- Orange
- Orangeville
- Orangeville
- Orchard Grove
- Orchard Heights Addition
- Orchard Highlands
- Orchard Park