His family emigrated to the United States and settled in Columbia, South Carolina, where his father's two brothers, Jeremiah and Joseph O'Connell, were serving as missionary priests.

[1] As a young man, O'Connell felt called to be a priest and entered St. Charles College in Ellicott City, Maryland, in 1868.

[2] In part due to the influence of his uncles, in 1871 he was sent to Rome to study at the Pontifical North American College.

"[3] O'Connell was ordained in Rome for the Diocese of Richmond on May 26, 1877, by Cardinal Raffaele La Valletta.

[4] Bishop Denis J. O'Connell High School in Arlington, Virginia, founded when the location was still part of the Diocese of Richmond, is named for him.