"It was a good win for us," Fisher said. "We played pretty well, for the most part. We were glad to be up 3-2, but we were making some mistakes, and they were feeding off that."
Andrej Meszaros, Chris Phillips, Dean McAmmond and Jason Spezza also scored for the Senators, who won for just the third time in their past eight games.
Martin Gerber turned aside 23 shots for his 23rd win this season.
"I saw the puck well tonight," Gerber said. "I think it was a good win for us."
The Islanders tried to come back from a 3-0 deficit, but goals by Sean Bergenheim and Bryan Berard weren't enough to prevent them from going 1-3-1 in their past five games.
All-Star goalie Rick DiPietro stopped 21 shots in the loss. New York is winless in five games at home (0-4-1).
The last time the Islanders went that long without a win was when they lost seven straight from Feb. 27 to March 29, 2001.
The Islanders have not won a home game in 2008 - their last win was Dec. 29, a 5-2 victory over New Jersey.
Ottawa got off to a 2-0 lead with a pair of goals in a 35-second span near the midway point of the first period.
On their first power play, the Senators went ahead at 9:18 when Meszaros' shot from the center point was tipped by Islanders defenseman Freddy Meyer and got past DiPietro.
Then at 9:53, Phillips walked in unimpeded from the point to the left circle and wristed a shot to the short side.
The Senators moved in front 3-0 in the second period when Fisher sent a blind backhand pass to the slot. McAmmond, alone in front, beat DiPietro with a short wrist shot. It was McAmmond's seventh goal this season, and came at 6:38.
The Islanders had a chance to break the shutout, but Blake Comeau hit the left post with a wrist shot at 8:16.
New York finally put one past Gerber at 15:58 when Josef Vasicek threw a centering pass from behind the net. Gerber tried to cut off the pass with his stick, but missed, and Bergenheim was all alone to send a wrist shot into the net.
Berard made it 3-2 at 6:35 of the final period when he and Miroslav Satan executed a perfect back-door play on New York's second power-play chance.
"They kind of turned it around after the power-play goal," Fisher said.
New York had a chance to tie the game at 8:18, but Richard Park's hurried shot on a break hit Gerber in the chest.
Ottawa clinched the game at 15:03, when Fisher finished a back-door play with his 19th goal.
"It was a 3-2 game," Islanders forward Mike Sillinger said. "We had control of the puck, and all of a sudden, it's in our net."
Spezza closed out the game with his 19th goal, an empty-netter, with 64 seconds left.
Sillinger said the Islanders have to show a bit more intensity in order to snap their home winless streak.
"We go on the road, and we're pumped up in another team's building," Sillinger said. "We have to show more desperation at home. We're finding ways to lose, and we have to turn it around quick."
