Oswald Tesimond, a Jesuit, was born in either Northumberland or York, in 1563 and was educated in York in ‘Le Horse Fayre’ free school along with Guy Fawkes and the Wright brothers John and Christopher. He entered the English College in Rome, on 9 September 1580 but joined the Society of Jesus on 13 April 1584 by leave of the cardinal protector, Moroni. After studying theology at Messina he taught philosophy there and at Palermo. It is not known when he was ordained but he was sent to the Madrid seminary, leaving in November 1597 to go on the English mission.

He landed at Gravesend on 9 March 1598. He assisted Father Edward Oldcorne in Worcestershire and Warwickshire for eight years. On 28 October 1603 he was professed of the four vows.

Tesimond is chiefly remembered for his role in the Gunpowder Plot. It seems certain that Robert Catesby revealed the plot to Tesimond, and Tesimond revealed it to the Jesuit superior, Henry Garnet, under the seal of confession about 23 July 1605 with the object of seeking his advice. Tesimond went to the conspirators on 6 November 1605 at Huddington (Worcestershire), after their flight from London, to give the usual consolations of religion to the conspirators seen as a group of Catholics in dire spiritual need. Thomas Wintour at his execution cleared the Jesuits and particularly Tesimond from any charge of counselling or advising in the plot. Although in his hand, Tesimond's so-called narrative of the Gunpowder Plot—in poor Italian but one of the two most complete accounts (both are at Stonyhurst College)—was probably based on the work in Latin of a secular priest. It was sent to Rome for the better information of the Jesuit authorities.

The proclamation for Tesimond's arrest on 15 January 1606 described him as

..of a reasonable stature, black hair, a brown beard cut close on the cheeks and left broad on the chin, somewhat long-visaged, lean in the face but of a good red complexion, his nose somewhat long and sharp at the end, his hands slender and long fingers, his body slender, his legs of a good proportion, his feet somewhat long and slender.

Successfully handing off an attempt by a pursuivant to arrest him in London he holed up in papist houses in Essex and Suffolk until he was able to take a small boat to Calais with a cargo of dead pigs, of which he passed as the owner. After some time at St Omer he moved south. Sir Edwin Rich reported his recent arrival in Naples in a letter to James I of 5 October 1610 warning him against accepting a gift of poisoned clothing which Tesimond was supposed to be sending him.

Tesimond became prefect of studies and consultor at Messina from 1617, a post he held for some years, with a spell at Rome from 10 January until 10 December 1621, when he returned to Messina. In 1626 he was in the professed house at Naples as confessor in the church. After holding other offices in the province he died in Naples on 23 August 1636 and was buried there.