O'Sullivan, a right-handed pitcher who was part of the Kansas City rotation in 2010 and '11, was dealt to the Blue Jays on Thursday in exchange for cash.
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.
O'Sullivan was obtained 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 KC.
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.