Ontario Hydro was the official name from 1974 of the Hydro-Electric Power Commission of Ontario which was established in 1906 by the provincial Power Commission Act to build transmission lines to supply municipal utilities with electricity generated by private companies already operating at Niagara Falls.
It soon developed its own generation resources by buying private generation stations and then becoming a major designer and builder of new stations. As most of the readily developed hydroelectric sites were exploited, the corporation expanded into building first coal fired generation and then nuclear power. It had become one of the largest, fully integrated electricity corporations in North America by the 1990s.
In 1999, Ontario Hydro was split up into five successor companies.
Read more about Ontario Hydro: History