There are 25 cases in Stanley Cup history where a team went up 3-0 only for the other team to win the next 2.
Famously there are only 4 cases where the comeback team ended up winning the series.
This would indicate a 16% chance of Ottawa taking the series.
Incidentally, we could also assume that Toronto has won 3, Ottawa 2, so Ottawa has a 40% chance per game to win, needs to win 2, so has a 0,4*0,4=16% chance of winning the series. Both approaches give the same answer.
So Ottawa has 16% statistically.