Ostapchuk was selected by the Vancouver Giants of the Western Hockey League (WHL) in the first round, 12th overall, in the 2018 WHL Draft. He played with the Giants for three seasons. After the Giants traded away their captain, Justin Sourdif, at the 2022 WHL trade deadline due to the team's poor play, Ostapchuk was named the team's new captain. The team turned around their fortunes and made the playoffs, where they surprised the league's number one team, the Everett Silvertips in the first round. They lost to the Kamloops Blazers in the second round. In his final season of junior, Ostapchuk split the 2022–23 season between Vancouver and the Winnipeg Ice.[1]