Nick Paul and Magnus Paajarvi scored, and Craig Anderson made 35 saves in his first start since Dec. 21 for the Senators (19-25-5), who lost for just the second time in six games (4-2-0). Anderson missed 11 games with a concussion and was the backup to Anders Nilsson in a 4-1 win at the Carolina Hurricanes on Friday.
"I thought overall it was pretty good," Anderson said of his performance. "Didn't get the result. We definitely want to get wins here and that's a big part. ... Overall, I felt like I was in shape and seeing the puck pretty good and battling. That was a big thing, just trying to get that timing back and it was there."
Gunnarsson gave the Blues a 3-2 lead at 12:16 of the third period off a pass from O'Reilly after Pat Maroon knocked the puck from under Anderson's glove. Ottawa challenged for goalie interference but the call on the ice stood.
"I got a little worried there when they were reviewing it," Gunnarsson said. "But good job. The forwards were reaching for pucks down there like crazy and that kind of made their team kind of sink in there and open it up."
Paajarvi tied it 2-2 for Ottawa with a shorthanded goal at 1:00 of the third when Zack Smith's centering pass deflected off his skate.
Dunn put the Blues ahead 2-1 with 2:32 remaining in the second period with a slap shot from above the left circle after an extended shift in the Senators zone.