Hall backhanded a rebound past Craig Anderson for his 19th goal as the Oilers improved to 5-2-1 in their last eight games.
"It's a good feeling in here getting a win like that, especially in overtime," said Gagner, who ended a five-game point streak. "They come hard and they played really well, so we're going to carry this momentum home."
Gagner had 15 points during the run, including an eight-point effort in an 8-4 win over Chicago on Feb. 2 that tied a team record held by Wayne Gretzky and Paul Coffey.
"Nik was really dialed in and saw pucks," Oilers coach Tom Renney said. "The three that got by him he had a hard time with, just because of traffic, but he really gave us a chance to win."
Anderson stopped 36 shots in his NHL-leading 50th start this season. His last two saves came on Hall's initial shot and Horcoff's off the rebound before Hall came around behind the net to put away the winner.
"I've got to do a better job of controlling my rebound," Anderson said. "I kind of threw it out to the slot there, and they got a second whack, and then a third whack. Anytime you give teams two or three opportunities, more times than not they're going to put it in the net."
Paajarvi scored his first goal in 35 games this season at 14:05 of the second. He set up Horcoff's 10th goal for his third point in two games since he was recalled from Oklahoma City of the AHL on Tuesday.
Karlsson, who assisted on Alfredsson's goal in the first, drew Ottawa even at 3 with his ninth goal 9:30 into the third.
Senators defenseman Chris Phillips was honored in a pregame ceremony for playing his 1,000th career game Thursday. He reached the milestone in style by scoring twice -- including the game-winning goal -- in a 4-3 victory over Nashville.
Over 1,000 Quebec Nordiques fans were on hand at Scotiabank Place, most sitting in the arena's upper bowl. Hopeful of getting their team back in the NHL, they chanted "Nordiques Nation" and stood and cheered when the clock reached 16 minutes in all three periods, a nod to the number of seasons the French-speaking city has been out of the league since the franchise was relocated to Colorado for the 1995-96 season.
"I heard something. I think I looked up, I saw blue, but I didn't know what it was," said Alfredsson, who scored his 19th goal 4:36 in, assisted by Karlsson and Spezza.
Paajarvi's goal 14:05 into the second tied it at 1.
"It's a huge weight off his shoulders," Gagner said. "He's got a lot of skill, so for him to get one it's going to really open up a lot of confidence for him and hopefully keep scoring here."
Michalek's 24th goal at 16:23 restored Ottawa's lead. Spezza took Bobby Butler's backward pass along the right side and fed a breakaway pass up the middle to Michalek at the Edmonton blue line. Michalek drove in and slipped a backhand under Khabibulin's left pad to add to his team goal lead.
- Video messages for Phillips from former Senators teammates during the first period included Boston captain Zdeno Chara, New Jersey's Anton Volchenkov, Wade Redden and Nashville's Mike Fisher. "It was pretty cool to see those guys up there," Phillips said. "I played alongside those guys for a number of years, so it was nice to see."
- It was Paajarvi's first multipoint game of the season. He had four assists through his first 33 games.