O'Brien has been Director of various criminal defense clinics at UMKC School of Law since 1983, including the Public Defender Appeals Clinic (1983-1985), the Public Defender Trial Clinic (1985-1989), and the Death Penalty Representation Clinic (1990-present). He is a member of the Board of Directors of the Midwest Innocence Project. O'Brien served as the Chief Public Defender in Kansas City, Missouri from 1985 through 1989, when he was appointed Executive Director of the Missouri Capital Punishment Resource Center, now the Public Interest Litigation Clinic, where he represents clients in capital trial, appeal and postconviction cases. Professor O'Brien is a Past President of Missouri Association of Criminal Defense Lawyers, former Chair of the Missouri Bar Criminal Law Committee, and is a frequent lecturer on criminal justice issues.