Selman was outrighted to Triple-A Las Vegas on Thursday, John Shea of the San Francisco Chronicle reports.
Selman spent the final two weeks of the season in the big leagues but is no longer on the 40-man roster. He appeared in 16 games for the A's last season and had a 4.91 ERA, 1.15 WHIP and 18:5 K:BB across 18.1 innings.