O
- (Simcoe County) Ogden's Beach
- Oil Springs
- (Lambton County) Oakdale
- (Frontenac County) Oak Flats
- (Renfrew County) Oakgrove
- (Northumberland County) Oak Heights
- (Peterborough County) Oak Lake
- (Brant) Oakland
- (United Counties of Leeds and Grenville) Oak Leaf
- (Thunder Bay District) O'Connor
- (Frontenac County) Oconto
- (Lennox and Addington County) Odessa
- (Oxford County) Oriel
- (Renfrew County) O'Grady Settlement
- (Parry Sound District) Ojibway Island
- (Manitoulin District) Old Spring Bay
- (Regional Municipality of Niagara) O'Reilly's Bridge
- (Grey County) Orchardville
- (Elgin County) Orchard Beach
- (Parry Sound District) Oranmore
- (Cochrane District) Opasatika
- (Frontenac County) Ompah
- (Wellington County) Olivet
- (Oxford County) Oliver
- (Bruce County) Old Woman's River
- (Ottawa) Old Stittsville
- (Simcoe County) Old Fort
- (Kawartha Lakes) Omemee
- (Ottawa) Osgoode
- Owen Sound