Batherson also had an assist, Derick Brassard and Scott Sabourin scored, and Magnus Hellberg made 18 saves for Ottawa, which finished the preseason 5-3-0.
Cole Caufield and Kaiden Guhle scored, and Jake Allen made 26 saves for Montreal, which did not win a preseason game (0-6-2).
Caufield opened the scoring with a power-play goal from the left face-off circle to give the Canadiens a 1-0 lead at 8:47 of the first period.
Brassard, on a professional tryout contract with the Senators, tied it 1-1 at 3:48 of the second period by poking in a rebound for his first goal in four preseason games.
Brassard had an assist on Batherson's game-winner.
Sabourin scored at 15:17 to put Ottawa ahead 2-1, beating Allen after initially losing his balance.
Guhle tied it 2-2 at 18:40.
