2010-11: Bo Horvat played in six games junior games, including one playoff contest, for the Greater Ontario league's St. Thomas Stars and led the Elgin-Middlesex Chiefs midget AAA team in scoring. He had 1 goal with 3 assists for St. Thomas and scored 30 goals with 31 assists and had 12 penalty minutes in 30 games for the Chiefs. He scored 5 goals with 7 assists in 12 midget playoff games. Horvat played for Team Ontario in the Canada Winter Games; scoring 4 goals with 5 assists in nine games. He was selected by London in the first round (ninth overall) of the 2011 OHL Priority Draft.
2011-12: Horvat skated in 64 of 68 games for the OHL champion London Knights as a rookie and played for Canada Ontario in the 2012 U17 World Hockey Challenge. He scored 11 goals with 19 assists and was plus-27 with 8 penalty minutes for London. The Knights finished with the league's best record and rolled through the playoffs. Horvat scored 1 goal with 3 assists in 18 playoff games and was plus-one with 1 goal in four games at the Memorial Cup. He was named captain of Canada Ontario at the WHC and scored 4 goals with 3 assists in six games.
2012-13: Horvat was the fourth-leading scorer for London after representing Canada in August at the 2012 U18 Ivan Hlinka Tournament. He scored 32 goals with 28 assists in 67 games and was plus-three with 29 penalty minutes. The Knights dominated the OHL, finishing with the league's best record for the second straight year and topping 100 points for the seventh time in the last 10 seasons. In London's first 13 playoff games Horvath scored 8 goals with 3 assists and was plus-four with 8 penalty minutes. Skating alongside London teammate Max Domi at the Ivan Hlinka Tournament, he scored 2 goals with 2 assists in five games as Canada captured a gold medal. Horvat was ranked 15th amongst North American skaters in Central Scouting's final rankings prior to the 2013 NHL Draft.