In 2005, Harden began the season with the Athletics, but was sidelined with an oblique injury, and missed more than a month. Harden came back and pitched a two-hit game against the Texas Rangers on July 14, in which he allowed no runs for 7+ innings. One month later on August 14, Harden allowed only one hit, but received a no decision, en route to a 2–1 loss against the Minnesota Twins. On August 19, Harden had a 10–5 record with an ERA of 2.63, until he injured his right shoulder, sidelining him until September 25, by which time the Athletics were already out of the playoff hunt. Harden appeared in three games late in the season, pitching 5 innings of shutout ball, striking out seven and walking one. He finished the year with a 10–5 win-loss record, an ERA of 2.57 and 121 strikeouts in 128 innings. He allowed only seven HR's all season long, and despite the injuries, he emerged as the ace of the Oakland Athletics pitching staff.