2009-10: Scott Kosmachuk skated for the Toronto Marlboros midget minor team in Ontario. The Marlies captured the championship at the 2010 OHL Showcase and in seven games Kosmachuk scored 1 goal with 2 assists and had 10 penalty minutes. He was selected by Guelph in the second round (29th overall) of the 2010 OHL Priority Draft.
2010-11: Kosmachuk played in all 68 games for Guelph in his first OHL season. He scored 6 goals with 15 assists and was plus-five with 25 penalty minutes. The Storm finished fourth in the Midwest Division. In the six-game first round series with Saginaw, Kosmachuk scored 1 goals and was plus-three with 5 penalty minutes.
2011-12: Kosmachuk had a breakout year offensively in his second season with Guelph and represented Canada at the 2012 U18 World Junior Championship. In 67 games with the Storm he scored 30 goals with 29 assists and was minus-10 with 110 penalty minutes. In Guelph's first-round series with Plymouth he scored 2 goals with 3 assists and was plus-one with 12 penalty minutes. In seven games with bronze medal-winning Canada at the U18 WJC, he was plus-one with 4 penalty minutes. He was invited to the NHL Draft Combine and ranked 24th amongst North American skaters in Central Scouting's final rankings prior to the 2012 NHL Draft.
2012-13: Kosmachuk was the leading scorer for Guelph in his third OHL season. He scored 35 goals with 30 assists and was +3 with 105 penalty minutes in 68 games. Guelph reached the playoffs after finishing fourth in the Midwest Division. Kosmachuk scored 1 goal and was -3 with 13 penalty minutes in the five-game playoff series with Kitchener.
Kosmachuk fits the mold of being an energy player with some offensive ability. He is a hard worker in the offensive zone, and is willing to go into the dirty areas of the ice. Kosmachuk can be difficult to knock off of the puck due to a lower center of gravity. He can be caught out of positon at times due to his exuberance, and in general must work on being competitive in all three zones. Kosmachuk could be a third- or fourth line energy player if he makes the transition to the pro game.
Kosmachuk is likely to spend another season in juniors before turning pro in 2014-15.