The letter “O” makes its triumphant return today. Thus, our saint is none other than Saint Oliver Plunkett:
Oliver Plunkett (also spelt Oliver Plunket) (Irish: Oilibhéar Pluincéid), (1 November 1625 – 1 July 1681) was the Roman Catholic Archbishop of Armagh and Primate of All Ireland who was the last victim of the Popish Plot. He was beatified in 1920 and canonised in 1975, thus becoming the first new Irish saint for almost seven hundred years.
A few snippets from the Wikipedia article on him:
On the enactment of the Test Act in 1673, to which Plunkett would not agree for doctrinal reasons, the college was closed and demolished. Plunkett went into hiding, travelling only in disguise, and refused a government edict to register at a seaport to await passage into exile. For the next few years he was largely left in peace since the Dublin government, except when put under pressure from the English government in London, preferred to leave the Catholic bishops alone.
In 1678 the so-called Popish Plot, concocted in England by clergyman Titus Oates, led to further anti-Roman Catholic action. Archbishop Peter Talbot of Dublin was arrested, and Plunkett again went into hiding. The Privy Council in London was told that Plunkett had plotted a French invasion. The moving spirit behind the campaign is said to have been Arthur Capell, the first Earl of Essex, who had been Lord Lieutenant of Ireland and hoped to resume the office by discrediting the Duke of Ormonde. However Essex was not a ruthless or unprincipled man and his later plea for mercy suggests that he had never intended that Plunkett should actually die.
You can find much more about this martyr at the link above.