They will also have a 26th man on hand, as will the Orioles, for the second game of Saturday's doubleheader, per league rules.
A's manager Bob Melvin said he was notified at 6:25 p.m. local time of Friday's rainout.
"They said it's going to rain all night and that we're playing two games tomorrow," Melvin said.
Left-hander Eric Surkamp, who made four starts for the A's in April before being optioned to Triple-A Nashville, is an option for Wednesday's matchup against the Red Sox. He would be pitching on six days' rest.
Baltimore will start right-hander Mike Wright in Game 1 on Saturday and right-hander Ubaldo Jimenez in Game 2.
Jane Lee has covered the A's for MLB.com since 2010. This story was not subject to the approval of Major League Baseball or its clubs.