The Athletics optioned Thomas to Triple-A Las Vegas on Friday.
The move makes room on the 26-man roster for Jonathan Ornelas, whose contract was selected by the Athletics from Las Vegas. Thomas has been on the big-league roster since late April, but in 16 games since July 1 he's gone 4-for-34 (.118) with one RBI and a 38.9 percent strikeout rate while operating in a reserve role. He'll head back to Triple-A, where he's slashing .309/.397/.574 with one steal, five home runs and 19 RBI over 78 plate appearances this season.