Harden, who did not make an appearance during spring training, started the season on the disabled list with a strained right shoulder. He was transferred to the 60-day DL on April 18.
Oakland made a couple moves before hosting the Arizona Diamondbacks on Friday night. It also recalled right-hander Fautino De Los Santos from Triple-A Sacramento, optioned right-hander Trystan Magnuson to Sacramento and designated left-hander Bobby Cramer for assignment.
Harden was 36-19 with a 3.42 ERA in 97 games, 89 starts, with the A's between 2003-08. He was traded to the Chicago Cubs and went 19-15 with a 4.00 ERA with the Cubs and Texas Rangers before signing with the A's as a free agent during the offseason.
A's manager Bob Melvin announced a one-time six-man rotation to protect Harden. Josh Outman, Gio Gonzalez, Brandon McCarthy (currently on the DL with a stress reaction), Trevor Cahill and Guillermo Moscoso will follow the righty.