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- Charlie O'Brien, C, 1985
- John O'Donoghue, P, 1963-1965
- Harry O'Neill, P, 1922-1923
- Harry O'Neill, C, 1939
- Blue Moon Odom, P, 1964-1975
- Curly Ogden, P, 1922-1924
- Jim Oglesby, IF, 1936
- Rube Oldring, OF, 1906-1918
- Omar Olivares, P, 1999-2001
- Steve Ontiveros, P, 1985-1988, 1994-1995
- Mike Oquist, P, 1997-1999
- Billy Orr, IF, 1913-1914
- José Ortiz, IF, 2000-2001
- Roberto Ortiz, OF, 1950
- Ossie Orwoll, P, 1928-1929
- Dan Osinski, P, 1962
- Darrell Osteen, P, 1970
- Bill Oster, P, 1954
- Dave Otto, P, 1987-1990
- Josh Outman, P, 2008-
- Bob Owchinko, P, 1981-1982
- Jack Owens, C, 1935
- Doc Ozmer, P, 1923
Read more about this topic: Oakland Athletics All-time Roster