2010-11: Nedeljkovic played for the Cleveland Barons bantam AAA team — posting a 1.97 goals against in 52 games. He was among the youth goalies invited to the 2011 Warren Strelow National Team Goaltending Camp run by USA Hockey.
2011-12: Nedljkovic appeared in two games with the Cleveland Barons U18 midget team and played goal for the Belle Tire U16 team that finished second in the Midwest and was third at the 2012 USA Hockey Tier 1 U16 Nationals. He had a 4.32 goals against and .889 save percentage with the Barons. Nedeljkovic had a 1.88 goals against and .903 save percentage in 18 regular season games for Belle Tire and posted a 2.50 goals against and .915 save percentage in four playoff games. He was selected by Plymouth in the sixth round (123rd overall) in the 2012 OHL Priority Draft.
2012-13: Nedeljkovic made his OHL debut with Plymouth as a 16-year-old — after spending time with the Metro Jets in the NA3HL — and gradually took over the starter's role for the Whalers. He was 19-2-1 with 2 shutouts in 26 regular season games for Plymouth; posting a 2.28 goals against and .923 save percentage. The Whalers finished first in the West Division and reached the Western Conference finals against OHL champion London. Nedljkovic appeared in all 15 playoff games. He was 9-5 with 1 shutout and had a 2.71 goals against and .908 save percentage. In nine NA3HL games with the Jets he had a 2.49 goals against and .933 save percentage. Nedeljkovic attended USA Hockey's Select 17 Festival in July, 2013.
2013-14: Nedeljkovic played for the USA U18 team in the 2013 Ivan Hlinka Memorial Tournament in August before returning to Plymouth for his second OHL season. Nedeljkovic was 3-1 in four games at the Ivan Hlinka Tournament, suffering his only loss in the gold medal game against Canada, and had a 2.99 goals-against and .897 save percentage. He was named to Team Cherry for the 2014 CHL/NHL Top Prospects Game.