- Orangeville
- Oak Dale
- Oak Ridge
- Oakington
- Oakington Manor
- Oaklyn Manor
- Oakmont
- Oaks of Harford
- Oakwood
- Old Baltimore
- Old Dixie Manor
- Old Fields
- Old York Manor
- Open Acres Manor
- Orchard Lakes
- Orchardview Estates
- Osborne
- Osborns Ruins
- Overview Manor
- O'Donnell
- Oak Lea
- Oak View Village
- Oak West
- Oakland Mills
- Oakwood Overlook
- Owen Brown
- Old Town
- Olivet Hill
- Orchard Hill
- Overton