The Athletics selected Bielak's contract from Triple-A Las Vegas on Sunday.
He'll provide Oakland will a fresh arm out of the bullpen heading into Sunday's series finale with the Tigers while he joins the Athletics' 28-man roster as a replacement for lefty Brady Basso, who was sent back to Las Vegas after starting in Saturday's 2-1 loss. After being acquired from the Astros in mid-May, Bielak gave up two earned runs across 5.2 innings over his three relief appearances before Oakland cast him off its 40-man roster two weeks following the trade.