Denis O'Day (MBBS, FRACP, FACS) received his undergraduate degree from Xavier College in Melbourne, Australia, in 1953, and his M.B.B.S. degree from the University of Melbourne Medical School in 1960. After an internship and residency in medicine and internal medicine in Melbourne, O'Day completed a resident in ophthalmology at the University of California, San Francisco, in 1970. He then completed a fellowship in corneal disease at the Institute of Ophthalmology in London from 1970-1972.

O'Day was appointed Assistant Professor of Ophthalmology at Vanderbilt University School of Medicine in 1972; he rose to the rank of Professor by 1977. In 1989, he was named to the Michael J. Hogan Professor of Ophthalmology endowed chair, and in 1992, O'Day became the George weeks Hale Professor and Chair of the Department of Ophthalmology and Visual Sciences.