"It was cold," Ryan said. "Every period it got colder, but it was worth every second. We had a blast."
The Senators (11-13-7), who also beat the New York Rangers on Wednesday, have won two games in a row for the first time since beating the Colorado Avalanche twice in the 2017 SAP NHL Global Series in Stockholm on Nov. 10 and 11.
Ottawa is 3-10-2 since returning from Stockholm.
"Hopefully it's going to give us some confidence," Senators center Derick Brassard said. "We know we can put on a winning streak."
Carey Price made 35 saves in his 10th consecutive start for Montreal since returning from a lower-body injury. The Canadiens (14-15-4) have lost four of their past five games (1-3-1).
They were outshot 38-28 and won 29 percent of the face-offs (19-of-65).
"We need more," Montreal coach Claude Julien said. "We need to create more and we need to create more from the inside. Some nights we have that and tonight we didn't. Until we figure that out and become more consistent, that's what you are going to see."
Pageau, who grew up across the Ottawa River in Gatineau, Quebec, scored at 14:55 of the second period.
Seconds after Senators forward Tom Pyatt got the puck back after a lost in-zone face-off, Pageau found space in the high slot and deflected Karlsson's low wrist shot from the right point past Price for his fourth goal of the season.
His goal would eventually become the game-winner.
"To score that goal was pretty special," Pageau said. "When your goalie doesn't give up one it makes it easier."
Karlsson's assist added to his record-setting night. His ice time (32:55) and eight blocked shots are NHL regular-season outdoor game records.
"It was greater than we could have hoped for," Karlsson said. "The atmosphere was amazing."
The Canadiens never found a way to use the energy in Lansdowne Park to their advantage.
Not only did Ottawa have a 1-0 lead, it had a 29-16 advantage in shots on goal after two periods. A turnover by Montreal center Jonathan Drouin in the defensive zone led to Ryan's breakaway goal at 17:02, giving Ottawa a 2-0 lead.
Thompson scored into an empty net with 11 seconds remaining.
"A couple times we tried to make plays when they weren't there, and a couple times we tried to put it deep and forecheck when we could have made plays," Montreal forward Max Pacioretty said. "It just seems like we're always on the wrong end of those decisions right now. It doesn't allow for any sustained pressure."
Limiting Montreal's pressure was the Senators focus.
"We made it our game plan to not necessarily get in there and be physical, but be in front of them and make them uncomfortable," Ryan said. "It paid off over the course of the game."
That's two games in a row that style has paid off for the Senators. It's the same style that helped them get to Game 7 of the Eastern Conference Final last season.
"The old-fashioned way, the fashion that we're accustomed to, tight defense leads to offense," Ryan said. "Sometimes people tend to think it's opportunistic hockey, but it worked for us last year. We identified it again this week that we have to get back to it, and it worked tonight."
