Dominik Kubalik scored, and Joonas Korpisalo made 26 saves for the Senators (11-13-0), who have lost three of four.
"Obviously, they came out real hard," Ottawa coach D.J. Smith said of St. Louis. "We turned over two or three in the first period that allowed them to have a lot of zone time, gain momentum, and they came out way sharper than we did. We should know that they're going to come out sharp, obviously [losing] four in a row and the coaching change. So, we've got to be better than that. And then the second period, just a couple of really bad coverages, and it's in the back of the net.”
Thomas gave the Blues a 1-0 lead at 8:20 of the first period when he went in alone on Korpisalo after taking a pass through the middle from Faulk, fended off Artem Zub and scored between the pads.
Thomas made it 2-0 at 5:17 of the second period, collecting Faulk's wrist shot from the blue line and scoring from the slot with a backhand shot.
Brandon Saad made it 3-0 at 8:42, finishing a one-timer after Nick Leddy outwaited Zub's sliding attempt before finding Saad alone near the right side of the net.
Buchnevich made it 4-1 Blues at 7:07 of the third period, finishing off Jordan Kyrou's pass from the slot off a give-and-go.
"I thought we were really solid, playing aggressive on the puck, playing fast, playing together," Kyrou said of the line with Buchnevich and Thomas. "We were really supporting each other. That's the way we want to play."