It's unclear where O'Sullivan will fit back into the Padres' rotation. He was scheduled to start on Sunday, but the Padres announced that newly acquired righty Ian Kennedy will get the call instead.
O'Sullivan has pitched out of the bullpen this season as well. In four appearances, the San Diego native is 0-2 with a 3.72 ERA.
Mikolas pitched two-thirds scoreless innings of relief on Wednesday. He walked and struck out one.
He just completed his fourth stint with the Padres but has worked just 1 2/3 innings and has yet to allow a run.
In 44 games for Tucson this season he is 3-1 with 21 saves and a 2.86 ERA.
Jamal Collier is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.