Craig Anderson made 25 saves for the win.
Ottawa has a four-point lead on the Bruins in the Atlantic Division.
With just six remaining home games, the Senators gave the Canadian Tire Centre crowd something to remember in what could be a preview of a first-round playoff matchup should the standings remain as they are.
Ottawa took a 3-1 lead early in the third period as Hoffman scored his 20th on the power play. It was Ottawa's first goal with the man advantage in three games.
The lead was short-lived. Less than two minutes later, the Bruins scored on the power play as Marchand picked up David Pastrnak's rebound to score his 30th, giving him back-to-back 30-goal seasons.
There were no goals in the second period, but the Senators clearly held the edge, outshooting the Bruins, 16-5.
Ottawa had a number of chances as the Bruins turned the puck over several times, but Rask was solid and got a little help when Dion Phaneuf rang a shot off the post.
The Senators got a strong start as they held a 2-1 lead after 20 minutes.
Trailing 2-0, the Bruins cut the lead in half as Bergeron took a pass from Torey Krug in the slot and quickly turned and beat Anderson with a wrist shot in the final minutes of the period.
Brassard opened the scoring at 1:21 as he managed to beat Rask with a turnaround shot through traffic, and Ottawa took a 2-0 lead just three minutes into the game as Pageau redirected Phaneuf's point shot.
Notes: Ottawa defensemen Fredrik Claesson and Jyrki Jokipakka, and center Christopher DiDomenico were healthy scratches. ... The Senators remain without Bobby Ryan and Chris Neil, who both have broken fingers. ... Bruins right wing Jimmy Hayes, defensemen John-Michael Liles and Joe Morrow, and left wing Matt Beleskey were healthy scratches. ... Adam McQuaid was back in the lineup for the Bruins. He needed more than 20 stitches Saturday after taking a skate blade to the neck.
Bruins: Host Detroit on Wednesday night.
Senators: Begin a three-game trip Wednesday night at Dallas.