Ovechkin's 30th goal of the year boosted Washington to its eighth win in 11 games against the Sabres and allowed the Capitals to leapfrog idle Philadelphia into the East's top spot with 103 points.
The Capitals forced overtime when Jason Arnott found the net with only 52 seconds remaining.
Ovechkin added a pair of assists, Mike Knuble scored twice and Alexander Semin contributed a goal and a helper for Washington, which got 34 saves from Michal Neuvirth.
Drew Stafford got a pair of goals and Jhonas Enroth stopped 30-of-35 shot for the Sabres, who hold the seventh seed in the East.
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