Tarnok (shoulder) was activated off the 60-day injured list and optioned to Triple-A Las Vegas on Saturday.
Tarnok had been out all season while dealing with a strained shoulder. He's made four starts for Las Vegas while on a rehab assignment, recording a 1.84 ERA despite an 8:10 K:BB. He'll remain in Triple-A as a regular member of the roster but could be an option for an A's rotation that lacks proven talent at some point in the second half.