Nemisz joined the OHL's Windsor Spitfires in 2006–07, recording 34 points in 62 games. He improved to 67 points in 68 games with the Spitfires the following year, earning the team award for dedication and determination. His season led the Calgary Flames to select him with their first round selection, 25th overall, at the 2008 NHL Entry Draft, who project him to become a power forward. Nemisz set career highs with the Spitfires in 2008–09, scoring 36 goals and 77 points in 65 games, while adding 20 points in 20 games as he helped his team capture the J. Ross Robertson Cup as OHL champions. He then went on to score a goal and six assists in the 2009 Memorial Cup as the Spitfires defeated the Western Hockey League's Kelowna Rockets to win the national junior championship.