Agozzino made his major junior hockey debut with Niagara of the Ontario Hockey League after he was chosen by Mississauga with the No. 15 pick in the 2007 OHL Draft before the IceDogs relocated. He scored 27 goals in his second season of 2008-09, second on the IceDogs, and earned an invite to the St. Louis Blues development camp.
Midway through 2009-10, Agozzino was named Niagara captain after Alex Pietrangelo was traded to Barrie and finished the season with team highs of 37 goals and 66 points. Despite the success, Agozzino went undrafted in the NHL and signed an amateur tryout with the Blues' American Hockey League affiliate in Peoria. He returned to Niagara in 2010-11 and led it in goals for two straight seasons, including 40 goals, 48 assists and 88 points to finish fifth in the OHL scoring race.