DeBrincat won it with a one-timer off a pass from Claude Giroux on the power-play. The goals were the forward's first for Ottawa since being acquired in a trade with the Chicago Blackhawks on July 7.
Shane Pinto had two goals and an assist, and Giroux had three assists for Ottawa. Mads Sogaard made 11 saves on 14 shots before being replaced by Antoine Bibeau (one save) at the start of the third period.
Jake Evans scored twice, and Rem Pitlick and Justin Barron each had a goal and an assist for Montreal, which has lost its first four preseason games. Cayden Primeau made 29 saves.
Evans gave the Canadiens a 1-0 lead at 16:54 of the first period when he finished a pass from Pitlick on a 2-on-0 following a Senators turnover.
Evans then made it 2-0 from the edge of the crease with 59 seconds left in the period after Mike Matheson lofted the puck to him from the right point.
DeBrincat cut it to 2-1 at 7:41 of the second period when he spun and scored with a wrist shot from the left face-off dot.
Pitlick pushed it to 3-1 at 14:49 on a one-timer from the right circle, but Pinto cut it to 3-2 with his own one-timer on a power play at 15:54.
Barron restored Montreal's two-goal lede on the rush at 10:33 of the third period, but Mathieu Joseph responded 20 seconds later to cut it to 4-3.
Pinto tied it 4-4 with a one-timer on the power play at 14:40.
