Anders Lee scored twice, and Oliver Wahlstrom and Kieffer Bellows each had a goal and an assist for the Islanders (6-10-5). Ilya Sorokin made 30 saves.
"I think it's just a sense of relief," Lee said. "It's been a tough stretch. We stuck together and we started to build on our game a little bit the last few nights, but we hadn't been able to get that extra point in overtime. It starts to wear on you quite a bit. So for us to come out tonight and get the win that we needed, play the right way for good stretches of time, it's exactly what we needed out of our group."
Alex Formenton had a goal and an assist, and Filip Gustavsson made 22 saves for the Senators (7-16-1), who were coming off a 3-2 shootout win at the New Jersey Devils on Monday.
"At the end of the day, you could tell some of the guys were a little tired there as the game went on," Ottawa defenseman Thomas Chabot said. "But I mean, at the end of the day, the main thing for us is really looking back at the five games in seven days we had. We've had a good stretch (3-2-0), and we're starting to turn things around, but obviously this is another learning game."