The 1974–75 OMJHL season was the first season of the Ontario Major Junior Hockey League; the league having been created following a split with the Ontario Hockey Association. Eleven teams each played 70 games. The Toronto Marlboros won the J. Ross Robertson Cup, defeating the Hamilton Fincups.
Toronto Marlboros defeat Kingston Canadians 4–3, 1 tie
Peterborough Petes defeat Oshawa Generals 4–1
Hamilton Fincups defeat St. Catharines Black Hawks 4–0
Sudbury Wolves defeat Ottawa 67's 4–3
Toronto Marlboros defeat Sudbury Wolves 4–3, 1 tie
Hamilton Fincups defeat Peterborough Petes 4–2
Toronto Marlboros defeat Hamilton Fincups 4–3