Dean O'Brien has been dean of the law school since 1988, before which he was associate dean. He has the longest continuous service at a single institution of any law school dean in the country, and in 2012 was named one of the most influential people in legal education in a nationwide survey conducted by the National Jurist. He teaches Personal Income Tax and in the past has taught Taxation of Business Entities, Constitutional Law, and Legal Research and Writing.
From 1977 to 1985, Dean O'Brien was a senior attorney in the Office of the Chief Counsel of the Internal Revenue Service, heading the northeast region's tax shelter program. He joined the faculty of New England Law in 1985.
Dean O'Brien served as chair of the Council of the American Bar Association's Section of Legal Education and Admissions to the Bar in 2011–2012 and on the Council's Executive Committee as immediate past chair in 2012–2013. He previously served as chair of the ABA’s Accreditation Committee and of its Independent Law Schools Committee. In 2014, the Section of Legal Education recognized him with its highest award, the Robert J. Kutak Award, for his work on behalf of legal education and the profession. He is a founding member and former president of the Consortium for Innovative Legal Education, Inc., a group of four independent law schools that offers joint study abroad programs, visit-in programs to provide geographic flexibility for students, and other creative programming and services for the four schools. He is also a former member of the civilian police review board, formally known as the Community Ombudsman Oversight Program, which was established in 2007 by Boston Mayor Thomas M. Menino to review accusations of serious misconduct against Boston police officers.
He has brought widespread recognition to the law school by attracting a broad range of distinguished visitors, including US Supreme Court Justices Harry Blackmun, Ruth Bader Ginsburg, Anthony M. Kennedy, Sandra Day O'Connor, Antonin Scalia, and Clarence Thomas; Chief Justice John G. Roberts, Jr.; Dr. Hans Blix; Attorney General Janet Reno; and US senators Scott Brown and John F. Kerry.