Ondrus began his hockey career by playing with the Swift Current Broncos of the Western Hockey League. While playing with the Broncos, Ondrus was a teammate of future Maple Leaf players Ian White and Jeremy Williams. Ondrus played in five total seasons with the Broncos, where he registered WHL career totals of 94 goals, 113 assists and 207 points. Ondrus was also named captain of Swift Current during the final seasons of his tenure there. Ondrus made his professional hockey debut with the Idaho Steelheads of the WCHL during the 2002-03 season, where he played four games and recorded 3 assists. Ondrus was returned to Swift Current for his fifth and final season after his stint in the WCHL was completed. Ondrus went undrafted when he became eligible for the NHL draft, but performed well at the 2003 Toronto Maple Leafs rookie camp, and earned a contract with the St. John's Maple Leafs. Following an encouraging rookie season with St. John's, where he scored 6 goals and 17 points in 60 games with the team, Ondrus was given a contract with the NHL Maple Leafs. Ondrus would remain with St. John's during the 2004-05 season, as the NHL was in a lockout due to a labour dispute. Ondrus totalled 7 goals and 18 points in that season with the AHL Maple Leafs.
Ondrus made his National Hockey League debut on Tuesday, March 7, 2006, against the Montreal Canadiens. Ondrus would go on to play in 22 NHL games during the 2005-06 season, recording no points and 18 penalty minutes. Ondrus would go to play in 19 NHL games over the next two seasons, recording 2 assists. Ondrus has primarily remained a part of the minor league organization, which has since become the Toronto Marlies. During the 2005-06 season, Ondrus had his best professional career to date, as he scored 12 goals and 29 points in 53 AHL games. Ondrus is currently an active member of the Marlies roster, and is the team captain.