Aaron Harang participant of 2003 Major League Baseball season
Oakland Athletics
In 2000, Harang was traded with minor leaguer Ryan Cullen to the Oakland Athletics for Randy Velarde. He played with the AA Midland RockHounds in 2001, starting 27 games with a 10–8 record and 4.14 ERA.
Harang made his Major League debut for the Athletics on May 25, 2002, against the Tampa Bay Devil Rays. He pitched seven shutout innings in his debut, with 10 strikeouts, to record the win. He started 15 games for the team that season, and also made 1 relief appearance. He finished 5–4 with a 4.83 ERA. In 2003, he started 6 games, with a 1–3 record and 5.34 ERA.Cincinnati Reds
During the 2003 season, Harang was traded to the Reds along with Joe Valentine and minor leaguer Jeff Bruksch for José Guillén. In 2004, he went 10–9 with a 4.83 ERA for the Reds.
In 2005, Harang led the team in wins (11), quality starts (19), innings pitched (211+2⁄3), and strikeouts (163), all of which were career highs. He became the first Reds pitcher with 200 innings in a season since right-hander Elmer Dessens led the staff with 205 innings pitched in 2001. He received votes for NL Pitcher of the Month in May after going 3–1 with 1.93 ERA over five starts.