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.