Harden made his major league debut on July 21, 2003, against the Kansas City Royals. He held the Royals to only one run on four hits in seven innings, earning a no decision. Through Harden's first five starts, he had a 3–1 record, with an ERA of 1.69. In his next two, he allowed six runs in four innings against the Toronto Blue Jays on August 21, and then eight runs in 2.2 innings on August 26 against the Boston Red Sox. He finished the season with a 5–4 record and an ERA of 4.46. Harden pitched in two games in relief with the Athletics in their Division Series matchup with the Red Sox. In his playoff debut on October 1, Harden earned the win, pitching a scoreless inning. However in his second appearance, he pitched only one-third of an inning, and allowed two runs and was tagged with the loss.