P.K. Subban was off after his stick clipped Mark Pysyk and, with two seconds left in the man advantage, Ott was at the doorstep to shovel in a rebound and give Buffalo (11-15-4) only its second win in eight games.
Sabres scoring leader Thomas Vanek limped to the bench after taking a Christian Ehrhoff point shot off the hip during a power play eight minutes into the middle frame and did not return. The team said he had an upper body injury.
The Canadiens' failure to clear the puck from their own zone led to two Buffalo goals in the opening period.
Ott jumped on a loose puck and wired a goal in off a goalpost from the slot at 7:47 and Myers had a point shot go in off a stick in front of Carey Price at 18:59.
The Canadiens had an 11-2 shot advantage in the second period but failed to score, with Jonas Enroth stopping Michael Ryder at the doorstep.
Pacioretty finally scored on Montreal's 28th shot 7:04 into the third as he slid a shot in from the edge of the crease after some clever work by Brendan Gallagher behind the net.
Armstrong, with his second goal of the season and second in two games, deked to the backhand to beat Enroth at 16:09.
Notes: The Canadiens moved Gabriel Dumont to centre on the fourth line and promoted Armstrong to the third unit a game after he scored his first goal of the season. . . Michael Blunden was returned to AHL Hamilton with Michael Ryder back from an injury. . . Tomas Kaberle sat out for Montreal, while Nathan Gerbe and John Scott were Buffalo's scratches. Andrej Sekera missed with an upper body injury. . . The ceremonial faceoff saw former world boxing champions Jean Pascal and Lucian Bute approach centre ice from opposite directions. They fight each other May 25 at the Bell Centre.