The son of Irish gentry, Oliver was educated by his Cistercian cousin, Patrick Plunkett, who took part in the 1641 rebellion. Ordained in Rome in 1654, Oliver stayed there to teach and review books for the Sacred Congregation of the Index. In 1669, he was named archbishop of Armagh and primate of Ireland. Over the next decade, he organized synods, visited dioceses, opened schools and confirmed thousands of Catholics. With a new wave of English persecution, he was forced into hiding in 1679. Arrested late that year, while visiting his dying cousin, Patrick, in Dublin, he was imprisoned until his execution in London on trumped-up charges of treason. Extant letters reveal a devoted and courageous prelate who embraced martyrdom.