In his first start since Dec. 1, Filip Gustavsson made 22 saves as Josh Norris, Nick Holden and Alex Formenton scored for the Sens (7-16-1). Anders Lee (2), Oliver Wahlstrom, Kieffer Bellows and Jean-Gabriel Pageau had goals for the Islanders (6-10-5) while Ilya Sorokin stopped 30 shots.
"We've played five games in seven nights and we just didn't have the same sharpness with our execution tonight," Senators head coach D.J. Smith said. "We had some chances but we just didn't put it in.
"At the end of the day, we didn't get the result we wanted."
After a scoreless first frame, the Isles took a 1-0 lead 3:00 into the second as Josh Bailey found Lee down low where his initial shot missed wide but the puck careened back to him off the end boards and he slid the puck past a recovering Gustavsson for his fifth of the season.
The Senators got on the board at 9:44 and it came on the power play. After being high-sticked by Scott Mayfield, Norris got the ultimate revenge by burying the tying goal on a one-timer from his usual spot in the right faceoff circle for his 11th of the year.
But the Islanders took a hold on the game with two quick goals; Wahlstrom's eighth came at 14:06 as the puck fell to him in front of the net and he beat Gustavsson with a nifty move before Lee's second of the night came 27 seconds later as he banged the puck home from the side of the net after Mathew Barzal had been stopped initially after Adam Pelech's original shot had missed high.
Bellows pushed the Islanders' lead to 4-1 at 6:26 of the third period before Holden got his first as a Senator as his intended pass for Alex Formenton hit the skate of Noah Dobson at 8:11 and went in.
"I was trying to bank it off his foot," Holden joked. "No, I was just trying to create chaos out front and luckily it went off the d-man's skate."
Former Senator Pageau pushed the lead to 5-2 at 12:26 as he buried a rebound after Mayfield had been stopped during 4-on-4 play.
Formenton scored short-handed with 4:06 to play as he beat Sorokin over his glove for his fifth of the season and the Senators' first shorty of the campaign.
"We just didn't have enough in the tank tonight," Smith said. "We have three days off ... we'll go back to what we did to be successful in those other games and we'll be much sharper."
Batherson pushed his point streak to four games as he had an assist on Norris' goal while Formenton tallied his second multi-point game of the season and third of his career.
"It was one of those nights," Thomas Chabot said. "We had some scoring chances that could have gone our way but at the end of the day, this is another game and we have to focus on what we've accomplished the past couple of games and the way we're building our game as a team."
Ottawa is back in action Saturday when they host the Lightning at Canadian Tire Centre.
