Okajima attended
Higashiyama High School and pitched in the 1994
Koshien National High School Tournament.
Yomiuri Giants Okajima was drafted in the third round in 1993 by the
Yomiuri Giants of the
Nippon Professional Baseball League. Okajima pitched mostly as a
setup man, but sometimes played the role of a starter until 1999. When he played with the Giants in Japan, Okajima chose to go by the nickname "Okaji" instead of "Hideki" because
Hideki Matsui had preceded him on the Giants. Okajima became the team's closer in 2000–2001, before returning to his setup role in 2002.
Hokkaido Nippon-Ham Fighters Okajima was traded to the
Hokkaido Nippon-Ham Fighters right before the start of the 2006 season for two position players, and was a valuable lefty setup man for the team, pitching in 55 games, with a 2.14 ERA. The Fighters won the championship that year. In Japan, Okajima had a career record of 34–32 with a 3.36 ERA.
Boston Red Sox Okajima became a
free agent in 2006, and signed a two-year, $2.5 million deal with the
Boston Red Sox. The deal included a $1.75 million club option for a third year. During
spring training in 2007, Okajima was asked what he felt about being relatively anonymous while his teammate and countryman
Daisuke Matsuzaka's every move was scrutinized. Okajima stated, "I'm willing to be a hero in the dark." Okajima allowed a home run to
John Buck on his very first pitch in the major leagues. It was the eleventh time in MLB history a pitcher gave up a home run on his first pitch. However, Okajima then proceeded to hold opponents scoreless for nearly two months until the Yankees scored a run off of him via a
fielder's choice on May 22. At the start of the 2007 Red Sox season, injuries and poor performances limited the effectiveness of anticipated setup relievers
Mike Timlin and
Joel Piñeiro. Okajima stepped into the role of primary setup man for closer
Jonathan Papelbon and quickly became one of the most dominant relievers in the major leagues. Through the first two months of the season, Okajima cemented himself as the top setup man in the Red Sox bullpen garnering the American League Rookie of the Month for April. Okajima's scoreless streak from April to May 2007 was the longest by a Red Sox left-hander since
Bruce Hurst 20 years prior. After getting a save in his first appearance during a Red Sox–Yankees series, Okajima compared the experience to winning the pennant. For all of his early success, Okajima remained relatively anonymous in his forays around Boston. While Okajima was taking a cab ride with his wife to a local restaurant, his cab driver gushed about the amazing performance of the Red Sox's newly acquired lefty without knowing that the reliever was in the cab with him. In Japan, Okajima was known as a gloomy man. However, Okajima revealed more of his humorous side in the United States. On May 17, 2007, Okajima faced the heart of the
Detroit Tigers order for the second time in one day to earn his second major league save. When Japanese reporters asked him about facing the same three batters in both games of the
doubleheader, he answered with a smile, "I did?" Okajima made the All-Star team as the winner of the
final vote, but did not play in the game; he is the third Red Sox player to make the team this way. Following the All-Star break, Okajima continued to pitch solid and reliable relief innings occasionally closing games for the Sox as they held their place at the top of the
AL East. However, the rigors of the MLB season began to catch up with Okajima in the later stages of the season and he began to struggle culminating in a career-high four-run meltdown in a game against the Yankees on September 14, 2007. Red Sox management decided to shut him down following this outing to allow Okajima to relieve the physical and mental fatigue of the longer American season. On his return during the final stages of the regular season, Okajima regained his sharp early season form, pitching scoreless frames to establish himself again as an integral part of the dominant Red Sox bullpen as they headed into the postseason. He was selected to the
2007 Topps Major League Rookie All-Star Team. The selection was the result of the 49th annual Topps balloting of Major League managers. in . Okajima's performance really returned in Game 2 of the
World Series against the
Colorado Rockies, in which he pitched 2⅓ perfect innings in relief of
Curt Schilling, striking out four at
Fenway Park. He became the first Japanese born pitcher to play in the World Series. He closed the 2007 postseason with a home run to
Garrett Atkins. Okajima helped win the opening game in the
MLB Japan Opening Series 2008. On December 3, 2010, he was non-tendered by the Red Sox, making him a free agent. However, both parties agreed on a one-year deal on January 2. Okajima did not make the 2011 Opening Day roster, starting the season in Triple-A Pawtucket. On April 18, he was called up to the Red Sox. On May 20, 2011, Okajima was
designated for assignment. On May 24, 2011, Okajima cleared waivers, and he accepted his assignment to the Red Sox's AAA affiliate, the
Pawtucket Red Sox. The
New York Yankees signed Okajima to a minor league contract on December 28, 2011. He also received an invitation to spring training. However, Okajima failed his physical at the beginning of spring training resulting in his release.
Fukuoka SoftBank Hawks Okajima agreed to terms on a one-year contract with the
Fukuoka SoftBank Hawks on February 26, 2012. He was granted his release from this team on January 29, 2013 to pursue opportunities in MLB.
Oakland Athletics Okajima inked a minor league deal with an invitation to spring training pending a physical on February 11, 2013. He was optioned to Triple-A to begin the 2013 season. He had a 2.16 ERA with 18 strikeouts and two walks in 16 2/3 innings during his tenure in Triple-A. He was called up to join the A's 25-man roster on May 17, 2013. He was designated for assignment on September 3, 2013.
Second stint with Fukuoka SoftBank Hawks Okajima returned to Japan in 2014, signing a one-year contract with the Fukuoka SoftBank Hawks.
Yokohama Bay Stars After the 2014 season Okajima considered returning to MLB but instead chose to stay in Japan, signing with the
Yokohama DeNA BayStars in December 2014.
Baltimore Orioles Okajima signed a minor league deal with the
Baltimore Orioles in February 2015. He was released on March 31, 2016. He retired on July 18, 2016. ==Pitching style==