Sean O'Sullivan

On March 17, 2018, O'Sullivan signed with the Southern Maryland Blue Crabs of the Atlantic League of Professional Baseball. He was released on May 8, 2018.

On May 23, 2017, the Washington Nationals signed O'Sullivan to a minor league deal. He was assigned to Triple-A Syracuse. He elected free agency on November 6, 2017.

On November 24, 2016, O'Sullivan signed a one-year, $1.1 million contract with the Nexen Heroes of the KBO League. On May 3, 2017, O'Sullivan was released by the Heroes.

On December 13, 2015, O'Sullivan signed a minor league deal with the Boston Red Sox. During his Red Sox debut, he pitched one inning, and had a 9.00 ERA. On May 10, 2016, O'Sullivan had his first Red Sox start, which he won pitching six innings and giving up four earned runs in a 13–5 Red Sox victory. He gave up six earned runs in 4.1 innings pitched against Houston in 1 10–9 Red Sox victory, but he was designated for assignment by the team that day.

O'Sullivan signed a minor league deal with the San Diego Padres in December 2012, and was called up for a few starts during the season. He also served for a short time in the bullpen. He was designated for assignment on August 19, 2013, and ultimately reassigned to the Triple-A Tucson Padres. O'Sullivan finished the 2013 season 0-3 with an earned run average of 3.96. He declared free agency on October 4.

On December 18, 2013, the Philadelphia Phillies announced that they had signed him to a minor league contract that included a spring training invitation. His contract was selected from the Lehigh Valley IronPigs on June 28 to start the second game of a doubleheader, and he was designated for assignment on June 29. He was called back up on August 7. He was once again designated for assignment on August 10 to create a roster spot for pitcher Jerome Williams. He was promoted again in September. He was outrighted off the roster on October 6.

In April 2012, O'Sullivan failed to make the major league roster and was placed on waivers by the Kansas City Royals. No other major league club acquired his contract and he was subsequently assigned outright to the Royals' AAA team, the Omaha Storm Chasers.

On June 21, 2012, O'Sullivan was traded to the Toronto Blue Jays for cash considerations and assigned to AAA, but was released at the end of the season.

On July 22, 2010 he and Will Smith were traded to the Kansas City Royals in exchange for Alberto Callaspo.

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 of the Pacific Coast League. When Santana was not able to make his next scheduled start on June 23, 2009 and was 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.

After being reassigned to the AAA level, 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.

O'Sullivan was drafted by the Los Angeles Angels of Anaheim in the third round of the June 2005 draft. He began his pro career with the Rookie-level Orem Owlz of the Pioneer League (baseball) in 2006. He compiled a 4–0 win–loss record with a 2.14 ERA in 71.1 innings pitched, and was selected a post-season All-Star. He was promoted in 2007 to the Class A Cedar Rapids Kernels of the Midwest League, where he pitched 158.1 innings, had a 10–7 win–loss record and a 2.22 ERA. O'Sullivan played in 2008 for the Rancho Cucamonga Quakes of the California League (Advanced Class A), and was 16–8 in 158.0 innings with a 4.73 ERA; he was a post-season All-Star pick. O'Sullivan began 2009 with the Arkansas Travelers of the Double-A Texas League, where he was 1–2 with a 5.30 ERA in 18.2 innings. He was promoted to the Salt Lake Bees of the Triple-A Pacific Coast League, where he went 6–4 with a 5.48 ERA in 69.0 innings.

O'Sullivan played baseball for Valhalla High School (El Cajon, California), near San Diego. In 2004, he went 11–1, with his only loss to El Capitan High School (in Lakeside, California) in the finals of the California Interscholastic Federation San Diego Section championship. He claimed the East County Californian Pitcher of the Year and Player of the Year awards that season.

Sean Daniel O'Sullivan (born September 1, 1987) is an American professional baseball pitcher who is currently a free agent. He previously played in Major League Baseball (MLB) for the Los Angeles Angels of Anaheim, Kansas City Royals, San Diego Padres, Philadelphia Phillies, and Boston Red Sox and in KBO League for the Nexen Heroes.