Ridly Greig scored for the Senators (6-2-0), who were coming off a 7-2 loss to the St. Louis Blues on Tuesday but have won four of five. Shane Pinto scored the lone goal in the shootout.
Matias Maccelli scored for the Islanders (3-3-1), who have alternated wins and losses through their first seven games. Semyon Varlamov made 18 saves.
Warren Foegele appeared to give Ottawa a 1-0 lead at 4:50 of the first period. However, after ruling his shot a goal on the ice, a video review determined that the puck hit off the crossbar after glancing off the shoulder of Varlamov.
Stephen Halliday then appeared to score for the Senators at 11:54, but the Islanders challenged the play for offside, and the call was overturned after another video review.
Maccelli did score for New York with 12 seconds remaining in the period to make it 1-0. He stole the puck from Pinto just inside the blue line, skated to the top of the right circle and beat Ullmark glove side with a wrist shot.
Maccelli extended his point streak to five games (two goals, four assists).
Greig tied the game 1-1 at 15:43 of the second period. Foegele took a stretch pass at the blue line from Artem Zub, skated in on a 2-on-1, and sent a saucer pass to Greig for a one-timer over the blocker of Varlamov.
Both the Senators and the Islanders had a power play in overtime but could not convert.
