A single-admission doubleheader was scheduled for Thursday starting at 4:05 p.m. Both teams originally had scheduled to be off Thursday.
Pittsburgh and Baltimore haven't played since Sunday. The Pirates played 20 games in 20 days through Sunday.
Baltimore ace Chris Tillman (3-1, 3.38) is slated to go up against Brandon Cumpton (0-1, 2.57) in the nightcap.
The rainouts are delaying the season debut of third baseman Manny Machado, who injured a left knee ligament on Sept. 23 and had surgery Oct. 14. He will come off the 15-day disabled list when the Orioles next play.
Baltimore will have to make a roster move to free a spot for first baseman Steve Pearce, signed to a major-league contract Tuesday.
Pearce was designated for assignment on April 22 and released five days later. The Orioles brought him back after first baseman Chris Davis was put on the disabled list with a strained left oblique.