Óliver Pérez Martínez (born August 15, 1981) is a Mexican professional baseball pitcher for the Washington Nationals of Major League Baseball (MLB). He previously played for the San Diego Padres, Pittsburgh Pirates, New York Mets, Seattle Mariners, Arizona Diamondbacks and Houston Astros. He has also competed for the Mexican national baseball team in the 2006, 2009, and 2013 World Baseball Classics.
San Diego Padres
Pérez was signed by the San Diego Padres as an amateur free agent in 1999. He made his debut with the Padres in 2002. Pérez did well for a rookie after being called up by the Padres in 2002, striking out over a batter per inning, but he suffered from control issues at times. He regressed somewhat in 2003.
Before the 2004 season, the team overhauled his pitching mechanics. His average of 10.97 strikeouts per nine innings was highest in the Majors (239 SO/196 IP); his 2.98 ERA was fifth in the National League (tied with Roger Clemens); and his 12–10 record could have been ever better with reasonable run support early in the season. Pirates' bats provided two or fewer runs in Pérez' 16 starts before All-Star break, causing him to post a 5–4 record with five no decisions despite a 3.24 ERA. In that season, Pérez pitched at least six innings and allowed three or fewer runs in 21 of his 30 starts (70%). Through this first three seasons, Pérez had compiled a 20–25 record with 474 strikeouts and a 3.86 ERA in 412.2 innings. His 239 strikeouts that year are currently the third-most in a season by a modern-day Pirate, trailing only Bob Veale's 276 in 1965 and 250 in 1964.
By contrast, Pérez's 2005 season was disappointing. He posted a 5.85 ERA in a season plagued by injuries. The Pirates were forced to place him on the disabled list on June 29 after he broke his toe kicking a cart following a loss to St. Louis. He missed two and a half months, returning in September to post a 4.58 ERA in 19.2 late season innings. Pérez had also lost significant speed off his fastball.
In 2006, Pérez opened the Pirates' season as the number one starter. On June 27, Pérez was sent to the Pirates bullpen after struggling through the first half of the season with an ERA over 6.00. On June 29, he was sent to the Triple-A Indianapolis Indians, with Tom Gorzelanny being called up to replace him in the starting rotation. 
New York Mets
On July 31, 2006, Pérez and Roberto Hernández were traded to the New York Mets in exchange for Xavier Nady. The Mets assigned Pérez to their AAA affiliate, the Norfolk Tides. He was recalled to the Mets' major league roster on August 26, 2006. After two subpar starts, Pérez threw a complete game shutout against the Atlanta Braves in the second game of a doubleheader on September 6, 2006. 
After the Mets lost two starting pitchers to injury in the final week before the playoffs started, they were forced to use Pérez in the playoff rotation. His first playoff start came in Game 4 of the NLCS, in which he picked up his first career postseason win. His second playoff start came in Game 7 of the 2006 NLCS vs the Cardinals, he went 6 strong innings allowing 1 ER, The Mets would eventually go on to lose the game 3–1. 
Entering the 2007 season, there were concerns about the Mets' pitching staff and whether Pérez could live up to his potential. However, Pérez finished the 2007 season 15–10 with a 3.56 ERA, striking out 174 in 177 innings pitched, while walking only 79.
In the 2008 season, Pérez was inconsistent, posting a record of 10–7 with a 4.22 ERA. He was also prone to big innings striking at any moment. An example would be in a start against the San Francisco Giants, he went 1⁄ 3 of an inning allowing 5 hits and 6 runs all of them earned. An example of his dominance would be his start against the New York Yankees on June 29, 2008. Pérez went 7 innings allowing just one run on a home run by Wilson Betemit striking out a season high 8 batters. After the firing of manager Willie Randolph, Pérez pitched better. At the request of Pedro Martínez and pitching coach Dan Warthen, he has changed his delivery to the plate. Instead of letting his head hang down when he makes his delivery, he now makes a bowing motion. He led the majors in walks, with 105. 
On February 3, 2009, the Mets signed Pérez to a three-year $36 million deal to return.  On May 6, Pérez was put on the disabled list due to patellar tendinitis in his right leg.  He returned to the rotation on July 8, 2009.
On August 26, 2009, Pérez was diagnosed with patellar tendinitis in his right knee and underwent undergo season-ending surgery. He finished the season 3–4 with a 6.82 ERA. 
On May 15, 2010, manager Jerry Manuel moved Pérez to the bullpen. Pérez refused a minor league assignment to work on his pitching despite both his poor play and repeated attempts by the Mets' front office. 
On June 5, 2010, the Mets placed Pérez on the 15-day DL due to a patella tendinitis of right knee. As Pérez was placed on the DL soon after refusing an assignment to the minor leagues a second time, the league investigated the timing of the DL stint, later clearing it.  After July 21, Pérez made only six appearances, all in relief.  Pérez finished the 2010 season with 0 wins, 5 losses, and a 6.80 ERA in just 46.1 innings pitched.
The Mets unconditionally released Pérez on March 21, 2011, still being responsible for the remaining 12 million dollars on his contract. 
On March 23, 2011, the Washington Nationals signed Pérez to a minor league contract.  Perez chose to join the Nationals because their pitching coordinator, Spin Williams, was his pitching coach with Pittsburgh. Pérez was assigned to the Harrisburg Senators of the Class AA Eastern League. He pitched to a 3.09 ERA in 15 starts, but did not receive a promotion. Williams suggested to Pérez that he should transition into a relief pitcher. 
On January 19, 2012, the Seattle Mariners signed Pérez to a minor league deal with an invitation to spring training. The contract contained a $750,000 bonus for making the Mariners 25-man roster and $250,000 in performance incentives.  Pérez made 22 relief appearances with the Tacoma Rainiers of the Class AAA Pacific Coast League,  and was promoted to the major league club on June 16 as a reliever.  In his first month back in the majors since 2010, observers took note of his improved fastball velocity and strikeout-to-walk ratio.  In 2012, Pérez went 1-3 with a 2.12 ERA with 29.2 innings in 33 games.
On November 3, Pérez signed a one-year, $1.5 million deal with up to $600,000 in performance bonuses. 
Second stint with the Nationals
On December 11, 2015, Pérez signed a two-year, $7 million contract with the Washington Nationals.  On April 24, 2016, with the Nationals trailing the Minnesota Twins after 15 innings, and with two outs and Nationals shortstop Danny Espinosa on second base, Pérez hit a bunt that appeared to be rolling foul, but Minnesota catcher John Ryan Murphy picked it up and threw it away while attempting a putout at first, allowing Espinosa to score the tying run from second. In the next inning, Washington right fielder Chris Heisey hit a leadoff home run to end the game, and Pérez was credited with the win.