Oates began teaching at the University of Massachusetts Amherst in 1968. He was a professor of history and the Paul Murray Kendall Professor of Biography there until 1997. He also wrote 20 books during his career. Many were biographies of 19th-century American historical figures. In the biographies of John Brown, Nat Turner, Abraham Lincoln, and Martin Luther King Jr., Oates examined the lives of men committed to the struggle for equality. He wrote that "[a]ll four were driven, visionary men, all were caught up in the issues of slavery and race, and all devised their own solutions to those inflammable problems [a]nd all perished, too, in the conflicts and hostilities that surrounded the quest for equality in this country."