He was known to people who knew of the Gunpowder Plot to destroy the Parliament of England and kill King James I; and although his involvement is unclear, he was caught up in the subsequent investigation.

[4] In early 1589 he went with Father Henry Garnet to the West Midlands, visiting Coughton, Warwickshire and settling at Baddesley Clinton.

Oswald Tesimond assisted him after 1596;[6] Father Thomas Lister, another Jesuit, also supported Oldcorne's mission but found the requirements of the covert life difficult.

[8] On 3 November 1601, Oldcorne went on a pilgrimage to St Winefride's Well at Holywell in north Wales to obtain a cure for a cancer of the throat.

Amongst this group were the priests Oswald Tesimond, Henry Garnet and John Gerard, as well as Jesuit brothers Nicholas Owen and Ralph Ashley.

[9] Oldcorne and Garnet were arrested by Sir Henry Bromley and held briefly at the castle at Holt in Worcestershire before being taken to the Tower of London.

He recounted under interrogation that on 8 November 1605 there arrived Tesimond from Robert Wintour's who told Mr (H)Abington and himself that "he brought them the worst news that they had ever heard, and they were all undone".

[13] Oldcorne was executed at Red Hill, Worcester, together with John Wintour, Humphrey Littleton and Ralph Ashley, his Jesuit brother colleague.

[2] He was hanged, drawn, and quartered; it is said that, as Oldcorne waited on the ladder to die, Ashley kissed his feet and said, "What a happy man am I to follow in the steps of my sweet father".