Los Angeles (15-11-6) is tied for third place with Seattle in the Western Conference, with the top five clubs earning a position in the MLS post-season. Two points from their final two games would ensure the Galaxy a playoff berth.
Montreal (13-12-7) remained in a three-way tie for third place in the Eastern Conference with Chicago and Philadelphia. New England is one point behind those three teams and is still very much alive in the playoff race.
Each team in MLS has two regular-season games remaining.
Opare, who played his college soccer at the University of Michigan, scored his first MLS goal. He drove a left-footed shot from the center of the penalty area past Montreal goalkeeper Evan Bush.
Brian Rowe made two saves to record the shutout.