The 1977–78 OMJHL season was the fourth season of the Ontario Major Junior Hockey League. The Fincups franchise moved back from St. Catharines to Hamilton after a temporary year away from the city due to lack of a suitable arena. The league featured a wealth of scoring talent during the season, with two players recording the league's highest single season point totals. Third season veteran Bobby Smith, edged 17-year-old rookie Wayne Gretzky for the Eddie Powers Memorial Trophy, scoring 192 points. Twelve teams each played 68 games. The Peterborough Petes won the J. Ross Robertson Cup, defeating the Hamilton Fincups.
Kitchener Rangers defeat Toronto Marlboros 3–2
Sault Ste. Marie Greyhounds defeat Kingston Canadians 3–2
Hamilton Fincups defeat Windsor Spitfires 4–1, 1 tie
Peterborough Petes defeat Oshawa Generals 4–1, 1 tie
Ottawa 67's defeat Sault Ste. Marie Greyhounds 4–3, 1 tie
London Knights defeat Kitchener Rangers 4–0
Hamilton Fincups defeat London Knights 4–2, 1 tie
Peterborough Petes defeat Ottawa 67's 4–3, 1 tie