- Obtuse Hill
- Old Baldy
- Oronoque
- Oronoque Hills North
- Oronoque Hills South
- Otter Rocks
- Owenoke
- Ox Hill
- Oakland
- Oakland Gardens
- Oakland Gardens
- Old Avon Village
- Oxford Village
- Oak Island
- Ore Hill
- Old Saybrook Center
- Otter Cove Estates
- Oxford Park
- Oakdale Manor
- Old Cobble Rocks
- Oronoke
- Outer Island
- Outer Thimble
- Outer White Top
- Overlook
- Occum
- Old Lyme Shores
- Osprey Beach
- Oswegatchie
- Oxoboxo River