Selman was optioned to Triple-A Las Vegas on Thursday, Martin Gallegos of MLB.com reports.
Selman spent just under two weeks in the major-league bullpen and will head back to the minors after he gave up four runs on four hits and no walks while striking out one in two-thirds of an inning Wednesday against Atlanta. Right-hander Adam Oller was called up by Oakland to take his place on the active roster.