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COG was formed in 1984 by an agreement among the Canadian CANDU-owning utilities Ontario Hydro (now Ontario Power Ge​​neration), Hydro-Québec and New Brunswick Power, plus Atomic Energy of Canada Limited (AECL).

In 1986 the offshore utilities that own CANDU units in Argentina and Korea joined COG. Following the accident at Chernobyl and recognition that cooperation and sharing of operating experience among nuclear utilities was essential to ensure the safe operation of all nuclear power plants worldwide, the Pakistan Atomic Energy Commission​ and Nuclear Power Corporation of India, owners/operators of a number of PHWR power reactors of CANDU design, joined COG in 1989. With construction of CANDU reactors at Cernavoda and Qinshan, the respective utilities in Romania and China joined COG in 1991 and 1999, respectively.

Under the original agreement Ontario Hydro was the administrator of COG reporting to a Directing Committee comprised of representatives of the four Canadian Members. In 1999 COG was registered as a not-for-profit corporation and a Board of Directors was appointed to replace the previous Directing Committee.

In 2001 after signing a lease agreement with Ontario Power Generation to operate the Bruce A and B nuclear generating stations Bruce Power joined COG as an independent Canadian member.

Today, Hydro-Québec​, though no longer operating a generating station, is a participant of the Information Exchange program.

The Nuclear Waste Management Organization (NWMO)​ joined​ COG as a participant in the Nuclear Safety and Environmental Affairs prog​ram in June of 2017.

AECL has had a historic role in COG and while AECL transferred its membership to Canadian Nuclear Laboratories in 2015, they have continued to be an important partner. In 2019, AECL rejoined COG as a participant in the NSEA program benefiting the COG collaboration model and in turn the Canadian and CANDU industries.