Member, New York Philharmonic, since 1966. Former member, New York City Ballet Orchestra, Saidenberg Little Symphony, and American Symphony under Stokowski. Performed Schubert and Dvorak Quintets with Guarnerius Quartet. Recordings on GM, Teldec, CBS, RCA, and Kapp. Participated in Marlboro Music Festival and many other chamber music festivals and in first performances of Gunther Schuller Quartet for Four Double Basses. Current faculty member of The Juilliard School (co-chair of Double Bass Department, 1992 - 2002) and Manhattan School of Music. Double Bass faculty, Mannes School of Music since 1992. Master classes at Peabody, New England Conservatory, Yale, and Tanglewood.
O'Brien's “Double Bass Notebook” was published by Carl Fischer in 2016.
Presidential Medal from Manhattan School of Music (2010); International Society of Bassists 2011 Special Recognition Award for Teaching.