Sean Daniel O'Sullivan (born September 1, 1987 in San Diego, California) is a Major League Baseball pitcher for the Kansas City Royals.
O'Sullivan made his major league debut against the San Francisco Giants on June 16, 2009, at AT&T Park, making a spot-start in replacement of the injured Ervin Santana. He won his debut after pitching seven innings, giving up one run on five hits and one walk, and striking out five. After the game, he was optioned back to the Salt Lake Bees. When Santana was not able to make his next scheduled start on June 23, 2009 and placed on the 15-day disabled list, O'Sullivan was recalled to the Angels for a second start in which he pitched five plus innings allowing three runs.
On July 28, 2009, O'Sullivan pitched a no-hitter for the Bees against the Sacramento River Cats. The only baserunner he allowed was via a walk.