Ben Chiarot scored his second goal of the game on a one-timer from the slot 58 seconds into overtime.
"We've had our ups and downs, but he's been fairly consistent," Canadiens coach Claude Julien said. "He's been a great addition. His size (6-foot-3, 225 pounds). His grit. You don't expect that from him, but he's already scored a couple or overtime goals."
WATCH: [All Canadiens vs. Capitals highlights]
Tom Wilson scored for Washington with two seconds left in the third period to tie it 3-3.
Lars Eller scored, and Braden Holtby made 28 saves for Washington (37-17-6), which has lost three straight and is tied with the Pittsburgh Penguins for first place in the Metropolitan Division. Ovechkin, who had not scored in five games, is one goal from becoming the eighth player to score 700 in the NHL.
"You know we make some strides in certain directions that are positive and then we fail in other areas. It's no secret, we are going through a struggle right now," Capitals coach Todd Reirden said. "We didn't play well enough to be able to get two points against really any team in this league, so we need to play better."