Student, Loras College, Dubuque, Iowa. Student, St. Mary's Seminary, Baltimore.
At the time of his death, he was one of oldest American Roman Catholic bishops. O'Neill was born in East Dubuque, Illinois and was ordained a priest on March 27, 1943. Pope Paul VI appointed O'Neill bishop of the Rockford Diocese on August 19, 1968 and he was consecrated a bishop on October 11, 1968.
O'Neill retired from the Rockford Diocese April 19, 1994. O'Neill died on April 27, 2013 at the age of 95.