The move adds depth to a Toronto pitching staff that has been hampered by a series of injuries this season. Most notably, Brandon Morrow was forced to the disabled list on June 12 after 13 starts -- including three shutouts -- due to a strained left oblique. Three days later, Kyle Drabek hit the DL with a small tear in the ulnar collateral ligament of his right elbow, and rookie Drew Hutchison, also nursing a right elbow injury, followed Drabek to the DL the next day.
Fellow right-hander Dustin McGowan is on the 60-day DL with multiple shoulder injuries and has yet to make a start this year.
After failing to make the Royals' staff in Spring Training, O'Sullivan, 24, was optioned to Triple-A Omaha, where he was 5-4 with a 6.75 ERA in 17 games including five starts.
The Royals acquired O'Sullivan with left-hander Will Smith from the Los Angeles Angels on July 22, 2010, in a trade for third baseman Alberto Callaspo. Smith is still pitching for Omaha after making three starts (1-2, 9.00 ERA) for Kansas City.
O'Sullivan's Major League record is 10-14 with a 6.13 ERA, including a 5-12 mark with the Royals.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.