MONTREAL -- The New York Islanders' fourth-string goalie showed no signs of being intimidated by a sold-out Bell Centre crowd.
Koskinen, who lost his first career start Tuesday in Toronto, turned aside Montreal's first four shots in the tiebreaker after stopping 37 shots during the game.
"I'm pretty confident about my shootout skills," said Koskinen, who was pressed into service against the Maple Leafs when Kevin Poulin suffered a season-ending knee injury in the pregame warmup on Tuesday.
"That was a great win for us. The guys played unbelievable in front of me and that made my job pretty easy tonight."
The Islanders failed to convert their first three attempts before Okposo shot past Alex Auld to hand the Canadiens' their third straight loss.
Okposo was happy that his goal gave the win to Koskinen, who is the fifth goalie to play for the Islanders this season after Dwayne Roloson, who was traded to Tampa Bay, and Rick DiPietro, Nathan Lawson and Poulin, who are all sidelined by injuries.
"He deserved it, especially last game, getting thrown to the wolves like that right in warmups," Okposo said. "To get your win in your second NHL game in Montreal is something pretty special."
Michael Grabner drew the Islanders even at 3 with his second goal of the game 4:08 into the third. Grabner's first goal of the game 17:10 into the second tied it at 2.
The 23-year-old rookie has five goals and three assists in his past six games.
"He's a dangerous player when he gets moving," Islanders coach Jack Capuano said.
"After two periods I thought we controlled the play pretty well," Canadiens coach Jacques Martin said. "A couple of breakdowns, they capitalized, and we need to be better in a couple of areas. I think there were elements of fatigue, some mental mistakes that were caused by fatigue."
Auld made 30 saves.
Matt Moulson hit the left post with 41.2 seconds left in the second after he jumped on a Montreal turnover in the slot. He missed another chance to give the Islanders their first lead at 12:45 when he put a backhand wide of the right post after receiving P.A. Parenteau's return pass on a 2-on-1 break.
Pacioretty scored his third goal in two games 14:10 in to put the Canadiens up 1-0.
Tavares drew the Islanders even at 16:35. The 20-year-old center snapped a shot past Auld for his 19th goal.
Montreal went up 2-1 when Pouliot got his first point in eight games with his 11th goal 5:49 into the second.
Plekanec scored his 19th goal at 18:08, shooting past Koskinen on a 2-on-1 break with Andrei Kostitsyn to put Montreal up 3-2.
Grabner tied it at 3 with his second of the game early in the third, tying Tavares and Moulson with his 19th goal.
- G Joel Martin, who has yet to play an NHL game, backed up Koskinen for a second straight game. He was recalled on an emergency basis from Bridgeport of the AHL on Tuesday.
- Canadiens D Hal Gill did not play because of an upper-body injury.
- Forward Mathieu Darche did not dress for Montreal.