Ontario was a province of Canada. Toronto was a major city there. The Seventh Doctor once found himself there at Christmas. Lake Ontario was also in the province. (PROSE: Christmas in Toronto) Sarah Jane Smith kept a vehicle license plate from the province in her attic. (TV: Invasion of the Bane)
In the 2020s, there was a talking cat in Ontario. (TV: Praxeus)
Behind the scenes
Toronto, Ontario was the birthplace of Doctor Who creator Sydney Newman. He later died there, as well.
Ontario was also home to TVOntario, one of the most faithful broadcasters of the 1963 version of Doctor Who in North America. They were responsible for commissioning Howard Da Silva's continuity announcements, which figured prominently in the early North American broadcasts of the Tom Baker era.