O'Neill served as a state representative and as Speaker of the House. He was then elected the State Attorney General in 1950, serving from 1951-1957, when he was elected to the governorship. O'Neill served until 1959. He was defeated for reelection due to his support of the controversial proposed "right to work" amendment to the Ohio Constitution. He served on the Ohio Supreme Court from 1960 until his death serving as an associate justice, then Chief Justice. He had the distinction of serving in all three branches of Ohio government during his career.