The Cardinals and Athletics faced off in back-to-back World Series in 1930 and 1931. The then Philadelphia Athletics won the 1930 series in six games; the Cardinals won the 1931 series in seven.
Notable players that played for both franchises are Enos Slaughter, Orlando Cepeda, Dick Allen, Mark McGwire, Dennis Eckersly, Jason Isringhausen, Matt Holliday.
The Cardinals and A's have a history as active trading partners. Some notable trades between the organizations:
1985: The Cardinals sent Joaquin Andujar to the Oakland Athletics for Tim Conroy and Mike Heath.
1990: The Cardinals send McGee to Oakland for Felix Jose, Stan Royer and Daryl Green.
1996: The Oakland Athletics traded Todd Stottlemyre to the St. Louis Cardinals for Bret Wagner (minors), Allen Battle, Carl Dale and Jay Witasick
And The St. Louis Cardinals traded Steve Montgomery to the Oakland Athletics for Dennis Eckersley.
1997: The St. Louis Cardinals traded Eric Ludwick, T.J. Mathews and Blake Stein to the Oakland Athletics for Mark McGwire.
2004: The St. Louis Cardinals traded Daric Barton, Kiko Calero and Dan Haren to the Oakland Athletics for Mark Mulder.
2009: The Oakland Athletics traded Matt Holliday to the St. Louis Cardinals for Clayton Mortensen, Shane Peterson and Brett Wallace.
Daric Barton had a 5 WAR season as a 24 year old. Even had Dan Haren not been included in the deal, sadly we still didn't get any value from the Mark Mulder trade.
Sorta makes up for the other times that we fleeced them.