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.
Hurricanes 4, Panthers 3:Brent Burns scored two goals, including the game-winner, in his Carolina Hurricanes debut, a 4-3 victory against the Florida Panthers at PNC Arena in Raleigh, North Carolina.
Andrei Svechnikov and Martin Necas each had a goal and an assist, Jaccob Slavin had two assists, and Frederik Andersen made seven saves for Carolina.
Rudolfs Balcers, Ryan Lomberg and Zac Dalpe scored, and Chris Tierney had two assists for the Panthers. Spencer Knight made 43 saves.
Burns, who was acquired in a trade with the San Jose Sharks on July 13, gave Carolina a 1-0 lead at 6:12 of the first period.
Svechnikov pushed it to 2-0 at 11:56.
Balcers scored 48 seconds into the second period to cut it to 2-1, and Lomberg tied it 2-2 at 4:52.
Dalpe then gave Florida a 3-2 lead with a power-play goal at 9:16, but Necas tied it 3-3 at 14:44.
Burns scored his second of the game at 9:00 of the third period for the 4-3 final.
Bruins 4, Flyers 0:Marc McLaughlin and John Beecher each scored twice for the Boston Bruins in a 4-0 win against the Philadelphia Flyers at TD Garden in Boston.
Patrice Bergeron, who signed a one-year contract on Aug. 8, had an assist in his preseason debut for Boston. Linus Ullmark made 21 saves.
Felix Sandstrom made two saves for the Flyers before being replaced at the start of the second period by Troy Grosenick, who made 20 saves.
McLaughlin gave the Bruins a 1-0 lead with a power-play goal at 7:09 of the second period, tipping a point shot from Jack Ahcan. He then made it 2-0 at 11:55 of the third when he poked in a loose puck at the side of the net.
Beecher extended the lead to 3-0 at 15:23 before scoring a short-handed, empty-net goal at 17:12 for the 4-0 final.
Bruins forward Fabian Lysell left the game in the first period with an undisclosed injury after he was checked into the boards.