Andersen made 24 saves in his first game for the Maple Leafs since March 19 because of a knee injury.
"I felt really good," Andersen said. "Some weird plays, some unfortunate bounces. Maybe that's rust. Maybe if I do a few things differently, those are saves. But overall, I felt really good."
Josh Norris scored with a one-timer nine seconds into overtime off a pass from Brady Tkachuk on a 2-on-1 to win it for the Senators.
"What a way to end the season, on a great pass from Brady," Norris said. "A great end of the year for the team too. We played so well down the stretch and that's very exciting. We're just happy with how it went, and we want to carry that into next year."
Parker Kelly scored in his NHL debut, Nikita Zaitsev had a goal and an assist, and
Filip Gustavsson
made 30 saves for Ottawa (23-28-5), which failed to qualify for the Stanley Cup Playoffs for a fourth straight season but went 9-2-1 in its final 12 games.
"I'm just proud of the guys the last little bit, certainly because you're out of it with a couple months to go," Senators coach D.J. Smith said. "For the most part, we played hard right until the end, win or lose. These guys are getting better and it's good to see."