The Senators went on a man-advantage when Montreal's P.K. Subban was penalized for holding Erik Condra at the side of the Canadiens' net with 19 seconds left in regulation.
With Ottawa on a 4-on-3 situation in the overtime, MacArthur got the puck in the right circle and fired a long-range wrist shot that beat Montreal's Carey Price inside the near post.
Condra, Colin Greening and Mark Stone also scored and Craig Anderson made 26 saves as the Senators won a fourth straight for the first time this season.
Daniel Briere had two goals and an assist for Montreal.
Brian Gionta also scored and Price finished with 30 stops.
Pot vending machine to debut
Pistorius testifies he didn't consciously pull trigger when he shot girlfriend