Hunter was signed by the Kansas City Athletics as an Amateur Free Agent in 1964, and he was on the roster the following year. The hurler would be named an All-Star in both 1966 and 1967, and the franchise relocated to Oakland, where he was their star pitcher.
"Catfish" went to his third All-Star Game in 1970, and from 1971 to 1974, he would never have less than 21 Wins in a season. The A's were juggernauts from 1972 to 1974, and it was Hunter who led the charge on the mound. In all three of those years, Oakland won the World Series, and Hunter was an important reason they got there. In 1972, he would have an ERA of 2.04 and finished fourth in Cy Young voting. He had an even better Cy Young finish in 1973, where he was third in the balloting, but in 1974, he won the elite prize, with a career-high of 25 Wins (league-leading) and would also finish first in the American League in ERA (2.49) and WHIP (0.986).
In the playoffs for Oakland, Hunter went 6-2 with 49 Strikeouts.
The 1974 World Series would see the end of Hunter in an Oakland uniform. He would sign with the New York Yankees as one of the first elite Free Agent signings of the modern era. The overall numbers for Catfish in Oakland would see him post a record of 161-113 with 1,520 Strikeouts.Hunter would enter the Baseball Hall of Fame in 1987, and Oakland retired his number 27 four years later. The franchise would also name him as part of their first Athletics Hall of Fame Class in 2018.