The Athletics selected outfielder Jai Miller from Triple-A Sacramento before Sunday's series finale against the Rangers.
It will be his first Major League appearance with Oakland.
Miller, 26, has spent the entire season with Sacramento, batting .276 with 32 homers and 88 RBIs in 110 games. The slugger ranked second in the Pacific Coast League in homers and fourth in slugging (.588), but also led the league in strikeouts (179).
The outfielder had his best month of the season in July, when he batted .318 with 12 home runs and 24 RBIs in 27 games.
A fourth-round pick by the Marlins in the 2003 First-Year Player Draft, Miller made his big league debut with Florida in 2008 and went 0-for-1 in just one ballgame.
He went on to hit .236 with one homer and four RBIs in 20 games with the Royals last season.