The interleague matchup between crosstown rivals known as the 'Battle of the Beltways' 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.