O
- (Regional Municipality of Waterloo) Orr's Lake
- (Ottawa) Ottawa South
- (Ottawa) Ottawa East
- (Regional Municipality of Niagara) Ostryhon Corners
- (Oxford County) Ostrander
- (Frontenac County) Oso
- (Lambton County) Osborne
- (Dufferin County) Orton
- (Parry Sound District) Orrville
- (Parry Sound District) Otter Lake
- (Simcoe County) Orr Lake
- (Simcoe County) Oro Station
- (Simcoe County) Oro
- (Hastings County) Ormsby
- (Simcoe County) Orkney Beach
- (Bruce County) Otter Creek
- (Hastings County) Otter Creek
- (Elgin County) Ormond Beach
- (Thunder Bay District) Ouimet
- (Wellington County) Oustic
- (Manitoulin District) Outer Duck Island
- (United Counties of Leeds and Grenville) Outlet
- (Algoma) Outlook
- (Ottawa) Overbrook
- (Peterborough County) Owenbrook
- (Oxford County) Oxford Centre
- (United Counties of Leeds and Grenville) Oxford Station
- (Ottawa) Orléans
- (Durham) Orono
- (Simcoe County) Oro Beach