The Senators owned a 2-1 lead heading into the final period, but Washington produced three goals in a span of 4:20 to win for just the second time in six games.
Dale Hunter is 2-3 as coach of the Capitals with both wins having come against Ottawa.
Nicklas Backstrom scored on the power play 9:45 into the third period to lift the Capitals into a 2-2 tie and Ovechkin put a slap shot past Craig Anderson with 6:10 remaining in regulation to give Washington the lead for good.
Michalek's goal was his 16th of the season, moving him within one of Toronto's Phil Kessel for the league lead.
Tomas Vokoun made 31 saves to get the victory.
Anderson stopped 39 shots in Ottawa's fifth loss in eight games.