[2] Oldcastle is first mentioned in two separate documents in 1400, first as a plaintiff in a suit regarding the advowson of Almeley church, and again as serving as a knight under Lord Grey of Codnor in a military expedition to Scotland.

[1] Over the next few years, Oldcastle held notable positions in the Welsh campaigns of King Henry IV of England against Owain Glyndŵr, including captaincy first over Builth Castle in Brecknockshire and then over Kidwelly.

[4] The marriage brought Oldcastle a number of manors in Kent, Norfolk, Northamptonshire and Wiltshire, as well as Cooling Castle, and from 1409 until his accusation in 1413 he was summoned to parliament as Lord Cobham.

[6] Oldcastle was a member of the expedition which the young Henry sent to France in 1411 in a successful campaign to assist the Burgundians in the Armagnac-Burgundian Civil War.

[6] Lollardy had many supporters in Herefordshire, and Oldcastle himself had adopted Lollard doctrines before 1410, when the churches on his wife's estates in Kent were laid under interdict for unlicensed preaching.

[7] But his friendship with the new King Henry V prevented any decisive action until convincing evidence was found in one of Oldcastle's books, which was discovered in a shop in Paternoster Row, London.

[7] He also said, similarly to the Iconoclasts of the Byzantine Empire and in contradiction of St John of Damascus, that the use of Christian art as an aid to prayer is, "the great sin of idolatry".

King Henry V was still anxious to find a means of sparing the life of his old comrade, and ordered a reprieve of forty days in the hopes that Oldcastle might repent his views.

The religious orders would be forcibly dissolved and the property of the Catholic Church in England was to be divided up among the Lollards and used to create a new ruling class.

[7] He is believed to have been privy to the Southampton Plot in July 1415, when the Lollards pledged support and Oldcastle stirred some movement in the Welsh Marches.

Some historians believed he was captured in the upland Olchon Valley of western Herefordshire adjacent to the Black Mountains, Wales, not far from the village of Oldcastle itself in his family's old heartlands.

On 14 December he was formally condemned, on the record of his previous conviction, and that same day was hanged in St Giles's Fields, and burnt "gallows and all".

[7] Although Shakespeare's drunken, morbidly obese, and hedonistic excuse for a medieval knight still remains "my old lad of the Castle", the stage character has many differences from the Lollard rebel leader.