William Nylander's Toronto was served an excellent opportunity to eliminate their rivals from the playoffs through a 5-on-4 power play in four minutes of overtime.
But Swedish star goalie Linus Ullmark held off and made eight saves in overtime.
Instead, Ottawa's Jake Sanderson was able to decide the game for the home team with just over two minutes remaining.
The visiting Toronto team entered the Canadian Tire Centre with a 3-0 lead in matches.
The home team, however, showed no signs of giving up and went into the first period break with a 2-1 lead after goals from Tim Stützle and Shane Pinto, while Nylander was credited with an assist on John Tavares' goal.
In the second period, the visitors managed to even the score. Ottawa took the lead again in the third, only to see Oliver Ekman-Larsson equalize for Toronto via an assist from Nylander.
Ullmark made a total of 31 saves.
Toronto thus leads the series, which is decided in best of seven, with 3-1.
