Oystrick was hired as the team's coach and GM after head coach Darcy Haugan and 15 other members of the Saskatchewan Junior Hockey League team died following a bus crash on April 6.
The 36-year-old, who played three seasons in the NHL, led Humboldt to a 21-13-2-1 record. The Broncos' 45 points are the fourth-most in the 12-team league.
The Broncos released a statement after Oystrick's resignation:
Assistant Scott Barney will take over as the team's interim coach and general manager. Per David Giles of Global News Canada, the 39-year-old played professional hockey for 19 seasons, including 27 NHL games. He was hired prior to the 2018-19 season.
Barney's first game leading the Broncos will take place at home on Wednesday against the Notre Dame Hounds.