Kyle Turris -- making his debut for Ottawa after being obtained from Phoenix Saturday for David Rundblad and a second-round pick -- set up Condra on the decisive goal, which broke a 1-1 tie.
Erik Karlsson added a power-play marker 10 minutes later and Condra finished the scoring with an empty-netter for Ottawa, which passed the Sabres for third place in the Northeast Division.
Zack Smith scored and Craig Anderson turned aside 22-of-23 shots for the Senators.
Jordan Leopold netted the only goal for the injury-plagued Sabres, who have dropped six of nine in December.
Ryan Miller made 35 saves for Buffalo.