According to Scout.com's Oakland Clubhouse:

Haven't seen this mentioned elsewhere, apologies if it was:

The Oakland A's announced the signing of catcher Alberto Castillo to a minor league contract on Monday night. Castillo is a major league veteran who has spent time with the Mets, Yankees, Cardinals, Giants and, most recently, the Royals. Castillo appeared in 34 games for KC this season before being released on August 5. He is a career .222 hitter, but he is a strong defensive catcher. Castillo gives the A's a veteran option on September 1 if they want to bring up a third catcher for the pennant drive. The A's have carried only two catchers all season, which has limited their ability to use back-up catcher Adam Melhuse as a pinch-hitter or designated hitter. Castillo was believed to be ready to re-sign with the Mets when he changed his mind to sign with Oakland.

You know what this means, right? Adam Melhuse, welcome to the designated hitter role.

And not a moment too soon.