He was educated in York, in the Royal School of William and Mary in the Horse Fayre, which cost no money to attend. Guy Fawkes. Edward Oldcorne and brothers Christopher and John Wright were among Tesimond's classmates, all of whom would become involved in the Gunpowder Plot. Tesimond was entered into the Society of Jesus in 1584, but not before he left the English College in Rome, into which he had been first initiated; to leave the College and come into the Society, he required special permission from the cardinal protector (who was Giovanni Morone, at the time), which he successfully obtained.
Later, among other things, Tesimond studied theology in Messina, where he afterwards taught philosophy. In November 1587, some time before he left on a mission to England,[clarification needed] he was ordained, but the exact date is unknown. As part of this mission, he arrived at Gravesend on 9 March 1598. For eight years he then worked with Edward Oldcorne in Worcestershire and Warwickshire before he was professed of the Jesuits' four vows on 28 October 1603.
He would later become somewhat involved in the Gunpowder Plot (see below). He effectively documented his experiences in a narrative which he wrote sometime after. He lived the later part of his life in various places, including Saint-Omer and Naples, where he died on 23 August 1636 at age 73.
Oswald Tesimond played a small role in the Gunpowder plot; while not directly involved, he knew of the plot from the confessional, and the motives of the conspirators. It is a near-certainty that the actual plot was divulged to Tesimond by Robert Catesby, someone who was deeply involved. Tesimond, after gaining the knowledge, proceeded to inform his Jesuit superior, Henry Garnet. He did this during confession, probably on or around 23 July 1605.
With the knowledge he possessed, Tesimond and Garnet decided to give religious consolations to the conspirators. They both did this despite the nature of the plot, which involved treason; obviously, their choice of keeping their knowledge secret constituted a violation of the law. He gave these consolations on 6 November 1605 at Huddington, after they had returned from London. It was perhaps a result of this kindness to the conspirators on Tesimond's part that saw Thomas Wintour, at his later execution, clear all Jesuits (Tesimond specifically and most especially), from any involvement, and from any prior knowledge.
Tesimond wrote a narrative based on the happenings of the Gunpowder Plot. Written in Italian, Tesimond's narrative probably comprises one of the most detailed and complete accounts of the plot itself. It may have been based on another Latin account of an unknown and unnamed secular priest. The narrative was sent to Rome to be studied in depth, with the hope it might reveal crucial information to the Jesuit authorities. Tesimond's arrest warrant was sworn out on 15 January 1606, describing him in detail:
..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.
— excerpt from the proclamation for Tesimond's arrest.
Tesimond was able to escape arrest. He evaded the constables of London, then made his way to safe houses in Essex and Suffolk. He then proceeded to sail to Calais among a cargo of dead pigs by passing as the owner of the load.
Tesimond, after escaping arrest, spent some years in Saint-Omer. Later, from the year 1617, Tesimond was prefect of studies and consulter in Messina. He held this position there for many years, among time spent in Rome during the most of 1621, and among other minor occupations. After some time there, he moved to Naples, Italy, where he would die in 1636. This was documented in a letter by Sir Edwin Rich to James I of England, which warned the king against accepting any gift he might receive, which might consist of poisoned clothing from Tesimond; in England, vigilance was still elevated after the events that transpired following the Gunpowder Plot.
- Henry Hawkes Spinks, Jr The Gunpowder Plot and Lord Mounteagle's Letter Kessinger 1902