Oldcastle, Sir John, 1378?–1417, English leader of Lollardry. He married the heiress of Lord Cobham in 1408 and was known as "the good Lord Cobham." Under the rule of Henry IV he performed valuable military service, especially in Wales, where he became a friend of the prince of Wales (later Henry V). His devotion to the teachings of John Wyclif brought upon him in 1413 condemnation for heresy. Oldcastle escaped from the Tower of London and was active in Lollard conspiracies until 1417, when he was captured and condemned. He was executed by hanging over a slow fire.