He was educated locally and later worked as a van boy before joining the Irish army.
O'Reilly first played competitive football during a golden age for the O'Tooles club.
In ten years from 1918 until 1928 he won nine county senior championship medals with the club.
Success at club level saw O'Reilly join the Dublin senior team and he made his debut during the 1918 championship.
Over the course of the next decade he enjoyed much success and won three successive All-Ireland medals between 1921 and 1923.