Corey Patterson's two-out double off Tyler Clippard (8-5) set up the winning hit. The next batter, Tejada, lined a pitch to center field to easily score him with the go-ahead tally and the O's hung on to post their season-high fourth straight win.
David Hernandez (3-6) was credited with the victory after getting the Orioles out of an eighth-inning jam and Alfredo Simon notched his ninth save of the year with a scoreless ninth.
Adam Jones got an RBI double to extend his hitting streak to eight games for the victors while starter Jeremy Guthrie held the Nats to three runs over six innings.
Washington starter Luis Atilano surrendered only two earned runs in seven innings of work.