O'Sullivan, who was out of options, had to clear waivers before he could be assigned to Omaha, where he's expected to be a starter. Although the 25-man roster had been finalized earlier, O'Sullivan technically was the 26th man until Wednesday's move.
"We like Sully," manager Ned Yost said. "He's young, he competes, he can go down there with [Vin] Mazzaro and those guys and get in the mix and if we need somebody, he's a guy that we know."
The Royals also placed pitchers Joakim Soria, Felipe Paulino and Blake Wood on the 15-day disabled list, backdated to March 26. Soria underwent elbow surgery on Tuesday, while Paulino has forearm tightness and Wood has an elbow ailment.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.