2011-12: Olivier LeBlanc played in Quebec midget AAA hockey as a 15-year-old; skating for the Seminaire St-Francois Blizzard. He scored 3 goals with 12 assists and 24 penalty minutes in 40 regular season games. The Blizzard reached the Quebec finals, falling to College Esther-Blondin four games to one and missing out on a trip to the Telus Cup. LeBlanc scored 1 goal with 4 assists and 6 penalty minutes in 20 playoff games. He was selected by St. John in the first round (11th overall) of the 2012 QMJHL Entry Draft.
2012-13: LeBlanc played for Saint John and was an assistant captain for Canada Quebec at the 2013 U17 World Hockey Challenge. LeBlanc skated in 60 regular season games for the Sea Dogs and scored 4 goals with 14 assists; finishing -32 with 41 penalty minutes. Saint John slipped to fifth in the Maritimes Division one season after winning back-to-back QMJHL championships. The Sea Dogs made the playoffs despite having the league’s third worst record and LeBlanc was -4 with 4 penalty minutes in the four-game series with Memorial Cup-bound Halifax. He scored 1 goal with 3 assists and 4 penalty minutes in six games at the WHC for Canada Quebec.
2013-14: LeBlanc skated in 56 games for Saint John in his second season — missing nearly a month after suffering an injury in February. An assistant captain and the leading scorer among Sea Dogs defensemen, he scored 7 goals with 26 assists and was -15 with 58 penalty minutes. Saint John missed the playoffs after finishing last in the Maritimes Division. LeBlanc was ranked 126th amongst North American skaters in Central Scouting’s final rankings prior to the 2014 NHL Draft.