Before playing in the NHL, Domi played 1 season with the St. Michael's Buzzers of the Ontario Junior Hockey League (OJHL) and 4 seasons with the London Knights of the Ontario Hockey League (OHL). On September 23, 2011, Domi made his OHL debut. He scored a hat-trick and an assist in a 8–0 win over the Saginaw Spirit. He helped the Knights win the OHL Championship in 2012 and also won a bronze medal with Team Ontario at the 2012 World U-17 Hockey Challenge.