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.