The rosters for each All Star team were announced today, and the Oakland A's are represented by Trevor Cahill. The A's pitcher lost a battle for the fifth starter spot this spring, but has emerged as a revelation, currently sitting with an 8-2 record and a 2.74 ERA. The problem for Cahill is that he is slated to pitch next Sunday, which makes him ineligible to pitch in the All Star Game.
Given the potential injury concerns for any pitcher going two days before, it's certainly reasonable to have such a rule. The problem for the A's, as mentioned by Susan Slusser in the link above, is that unless his roster spot is replaced with another Oakland Athletic, the team will have nobody who could potentially appear in the game. Cahill is still considered an All-Star, even though he won't appear. However, because he's considered an All Star, the mandatory team representative is made.
Slusser mentioned Bailey and Suzuki as alternatives, but I'd be pretty surprised if the A's got a second rep. They deserve it, but given the way they fly under the radar, it'd be a surprise if any other Oakland A made the roster. Nonetheless we'll keep an eye out for the replacement.