Oreskovic was drafted 82nd overall by the Toronto Maple Leafs in the 2005 NHL Entry Draft. Oreskovic began his hockey career by playing with the Brampton Battalion of the Ontario Hockey League. In four seasons in the OHL, with the Battalion and the Owen Sound Attack, he scored 7 career goals and recorded 41 assists. Oreskovic was also named the OHL's Best Defenceman and Best Bodychecker in the year-end Eastern Conference Coaches' poll in 2005-06.
Oreskovic joined the Maple Leafs' AHL affiliate, the Toronto Marlies, in 2006-07, playing in 3 games and recording 1 assist. Oreskovic was then sent to the Columbia Inferno of the ECHL for the 2007-08 season, but returned to the Marlies to finish the year. Oreskovic played 54 AHL games in 2007-08, scoring 1 goal and recording 9 assists.