- Oak Island
- Oakdale
- Oakdale Manor
- Oakland
- Oakland Gardens
- Oakland Gardens
- Oakville
- Obtuse Hill
- Occum
- Old Avon Village
- Old Baldy
- Old Cobble Rocks
- Old Greenwich
- Old Lyme
- Old Lyme Shores
- Old Mystic
- Old Saybrook
- Old Saybrook Center
- Oneco
- Orange
- Orcutts
- Ore Hill
- Oronoke
- Oronoque
- Oronoque Hills North
- Oronoque Hills South
- Osprey Beach
- Oswegatchie
- Otter Cove Estates
- Otter Rocks