Sean Daniel O'Sullivan
- Bats Right, Throws Right
- Height 6' 1", Weight 220 lb.
- School Grossmont College
- High School Valhalla High School (El Cajon)
- Debut June 16, 2009
- Final Game July 8, 2016
- Born September 1, 1987 in San Diego, CA USA
Sean O'Sullivan was drafted by the Los Angeles Angels of Anaheim as a draft-and-follow pick in the 3rd round of the 2005 amateur draft. After one year of junior college, he was signed by scout Tim Corcoran for $500,000 and made his pro debut in 2006.
O'Sullivan won his major league debut, pitching for the Los Angeles Angels of Anaheim against the San Francisco Giants on June 16, 2009. He gave up one earned run in 7 innings, striking out 5 and walking one in an 8-1 victory on "Irish Heritage Night" in San Francisco. He had been called up earlier that day to replace an ailing Ervin Santana, then was sent down after the game to make room for reliever Rafael Rodriguez. O'Sullivan had started the 2009 season at AA Arkansas, then was promoted to AAA Salt Lake City where he was 5-2 in 10 starts at the time of his call-up.
In 2010, O'Sullivan made his first appearance of the season for the Angels on July 20th and went six innings at Yankee Stadium, giving up only two hits and two runs in win over the New York Yankees. Two days later he was traded with minor leaguer Will Smith to the Kansas City Royals for former Angel farmhand Alberto Callaspo.
O'Sullivan earned his first win in four years on May 17, 2015, pitching for the Philadelphia Phillies. He allowed no runs over 6 innings in a start against the Arizona Diamondbacks that day, getting credit for a 6-0 win after losing his first two decisions of the season. his last win had come as a member of the Royals back in 2011. He had since pitched for the San Diego Padres in 2013 and for the Phillies in 2014 after spending 2012 in the minors, going a combined 0-3 in limited action.
O'Sullivan's brother, Ryan O'Sullivan, has played in the minors since 2011.