The interleague matchup between crosstown rivals will be made up on Aug. 4.
The Orioles came into the game having won seven of eight. They activated right-hander Bud Norris from the disabled list to make his first start since June 21. He had been sidelined with a strained groin.
Doug Fister was scheduled to start for Washington. The Nationals lost the opener of the series 8-2 in 11 innings on Monday.
The home-and-home series was slated to move to Baltimore on Wednesday. The Orioles own a 27-20 advantage in the series.