She began her studies with Milton Kestenbaum, former principal bass of the Pittsburgh Symphony under Fritz Reiner and member of the NBC Symphony under Arturo Toscanini; and with Herman Reinshagen, assistant-principal bass of the New York Philharmonic under Gustav Mahler and Arturo Toscanini at the University of California, Los Angeles.
[citation needed] O'Brien attended the Music Academy of the West summer conservatory in 1952, 1953, 1954, and 1955.
[4] Before joining the New York Philharmonic, O'Brien performed with the New York City Ballet, the Metropolitan Opera, and the American Symphony Orchestra (under Leopold Stokowski, under whom she played the double bass solo as principal bass in the U.S. premiere of Alberto Ginastera's Variaciones Concertantes, in 1962).
She formerly taught at YMHA, the Estherwood Summer Musical Festival, and the Institute de Haute Etudes Musicales in Montreux, Switzerland.
O'Brien has given masterclasses at the Peabody Institute, the Tanglewood Festival, New England Conservatory, and Yale University.