I have already purchased the Canadian Electrical Code. Do I need the Ontario Electrical Safety Code?

Yes. The law requires that all electrical installations in Ontario meet the requirements of the Ontario Electrical Safety Code. If you perform electrical work in Ontario, you should have a current copy of the 2021 Ontario Electrical Safety Code. 

The 2021 edition of the OESC includes the Canadian Electrical Code, Part I plus several Ontario-specific amendments.  This new edition includes changes designed to enhance electrical safety and protect workers and the public.