Ross Detwiler dug an early six-run hole for the Nationals, who tried to mount a furious rally but came up just short against the Orioles.
Washington came back late but Jim Johnson closed them out in the ninth for the Orioles' second win of the series.
Right-hander Chien-Ming Wang, who is scheduled to come off the DL next week, will likely rejoin the Nationals as a reliever.
Struggling to score runs, manager Davey Johnson has benched Danny Espinosa and moved Ian Desmond to the No. 5 spot in his lineup tonight. Join the conversation here as the Nats take on the O's.