The moves came before Thursday's game against the Chicago Cubs.
The 22-year-old O'Neill is a top prospect. In 12 games with Memphis, the muscular Canadian batted .388 with six homers, 18 RBIs and an .837 slugging percentage. The sizzling start came after he pulled a hamstring during spring training.
O'Neill gives the Cardinals another position player while outfielder Tommy Pham rests a groin injury.
Manager Mike Matheny says O'Neill will make his major league debut soon. He adds that he ''wouldn't hesitate to throw Tyler in just about any spot in the outfield and get him an at-bat in a big situation.''