2010-11: Ondrej Kase spent his first full season with KLH Chomutov in the Czech Republic; appearing in eight games for the club’s U18 team and playing for the U16 team. He scored 3 goals with 3 assists and was +3 with 2 penalty minutes in U18 play. Kase scored 25 goals with 15 assists and was an even plus/minus with 12 penalty minutes in 30 U16 games.
2011-12: Kase skated for KLH Chomutov in the Czech U18 league and played for the Czech Republic in the 2012 U17 World Hockey Challenge. In 38 regular season games he scored 18 goals with 26 assists and was +24 with 14 penalty minutes with KLH Chomutov. In two playoff games he had 2 assists. Kase had no points and 4 penalty minutes in five games at the World Hockey Challenge.
2012-13: Kase made his pro hockey debut skating with SK Kadan in the Czech Extraliga B division after skating for KHL Chomutov’s U20 and U18 teams and represented the Czech Republic in three international tournaments. In 13 games (including four playoff contests) with SK Kadan he scored 3 goals with 1 assist and was +1 with 2 penalty minutes. Kase scored 9 goals with 7 assists in 25 games and was +12 with 20 penalty minutes in 25 U20 games and had 10 goals with 16 assists in 14 games with KLH Chomutov’s U18 team. He was scoreless in four games with the Czech U19 team at the 2012 World Junior A Challenge. Kase played for the Czech U18 team in the 2012 Ivan Hlinka Tournament and the 2013 World Junior Championship. He scored 4 goals with 4 assists and was -6 in ten games.
2013-14: Kase made his debut with KLH Chomutov’s Czech Extraliga A Division club during the European Trophy tournament in the summer and played for the Czech Republic in the 2014 World Junior Championship. In five games at the WJC he was -3 with 1 goal and 2 assists. Kase was ranked 16th amongst European skaters in Central Scouting’s mid-term rankings prior to the 2014 NHL Draft.