After Nick Markakis led off the seventh with a single, Guerrero lined an 0-2 pitch from Nats starter Jordan Zimmermann (2-5) into left field that just cleared the fence for a 2-1 O's lead.
Orioles starter Chris Tillman held Washington to one run on six hits over five innings and Jim Johnson (3-1) was credited with the victory.
Zimmermann took the hard-luck loss, yielding only Guerrero's homer among four hits in 6 1/3 frames.
Alex Cora provided the Nats' only RBI.
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