O
- O'Briens Landing
- O'Connell
- O'Donnell Landing
- O'Grady Settlement
- O'Reilly's Bridge
- Oak Flats
- Oak Heights
- Oak Lake
- Oak Lake, Peterborough County
- Oak Leaf
- Oak Ridges
- Oak Shores Estates
- Oak Valley
- Oakdale
- Oakdene Point
- Oakgrove
- Oakhill Forest
- Oakland, Brant County
- Oakland, Essex County
- Oaklawn Beach
- Oakville
- Oakwood
- Oba
- Ochiichagwe'Babigo'Ining
- Oconto
- Odenback
- Odessa
- Off Lake Corner
- Ogden's Beach
- Ogoki
- Ohsweken
- Oil City
- Oil Springs
- Ojibway Island
- Ojibways of Hiawatha First Nation
- Old Cut
- Old Fort
- Old Killaloe
- Old Spring Bay
- Old Stittsville
- Old Woman's River
- Oldcastle
- Oldfield
- Olinda
- Oliphant
- Oliver, Essex County
- Oliver, Middlesex County
- Olivet
- Omagh
- Omemee
- Ompah
- Onaping
- Onaping Falls
- Onondaga
- Onyotaa:ka First Nation
- Opasatika
- Opeongo
- Ophir
- Orange Corners
- Orangeville
- Oranmore
- Orchard Point
- Orchardville
- Orillia
- Orleans
- Ormsby
- Oro Station
- Oro–Medonte
- Orono
- Orton
- Osaca
- Osborne
- Osceola
- Osgoode
- Oshawa
- Oso
- Osprey
- Otonabee-South Monaghan
- Ottawa
- Otter Creek, Bruce County, Ontario; Otter Creek, Hastings County
- Otterville
- Ouellette
- Oustic
- Outlet
- Owen Sound
- Oxenden
- Oxford
- Oxford Mills
Read more about this topic: List Of Communities In Ontario