Robert Marchant O'Neil (October 16, 1934 – September 30, 2018) was an American lawyer and educator. A specialist in constitutional law, he served as president of the University of Virginia (1985-1990) and created the Thomas Jefferson Center for the Protection of Free Expression, whose director he became (1990-2010). During these years, O'Neil remained associated with the university's law school, where he taught courses in the First Amendment and the Arts, Speech and Press, Church and State, and Free Speech in Cyberspace.