The opening two rounds will be split between the two courses, with the top 70 players and ties advancing to the final two rounds at Mickelson National.
Twelve players ranked inside the top 500 of the World Amateur Golf Ranking are in the field, with competitors representing 10 countries.
Mickelson National will host its second major national championship, having previously hosted the PGA Championship of Canada in 2024.
The winner will receive the Earl Grey Cup and earn exemptions into several major events, including the 2027 RBC Canadian Open.
The 72-hole stroke play championship begins Monday, July 27, with the final round set for Thursday, July 30.
The event will also feature the Inter-Provincial Team Championship, with Team Ontario looking to defend its title.