French cardinal, diplomat, and writer, b. at Larroque-Magnoac (Gascony), 20 July, 1537; d. at Rome, 13 March, 1604, was the son of a blacksmith. He was sent to the College of Auch as tutor to the sons of a nobleman, then to Paris, where he became the pupil and friend of the famous Ramus, whom he defended in two pamphlets against Charpentier, rector of the university. He next studied law at Bourges under Cujas and became an advocate before the Parliament of Paris, while acting as tutor to Jean de la Barrière, the future reformer of the Feuillants. In 1572 he joined the household of Paul de Foix, Archbishop-elect of Toulouse, whom he accompanied on various embassies and finally to Rome. De Foix dying in 1584, d'Ossat remained at Rome, supervising the French embassy for a year, and then becoming secretary successively to Louis d'Este and Joyeuse, two cardinal protectors of the interests of France. In 1588 he refused the post of minister of foreign affairs to Henry III. Driven from Rome by the rupture of diplomatic relations after the murder of Cardinal de Guise (1588), he returned after the death of Henry III (1589) as the private agent of his widow, Louise de Vaudemont. He used his position to support the cause of Henry IV, whose conversion he prepared the pope to accept. As agent for that prince, co-operating with du Perron, he negotiated the reconciliation with the pope, which took place 19 Sept., 1595. This was the greatest act of d'Ossat's diplomatic career, assuring as it did the definitive triumph of Henry IV over the League, and the restoration of peace and prosperity to France after more than thirty years of civil war. D'Ossat was appointed Bishop of Rennes (1596), cardinal (1589), and finally Bishop of Bayeux. Remaining at Rome, without any well-defined office, he was charged with occasional missions to Venice and Florence (1598), or managed the French embassy in the absence of the ambassador, and was always the enlightened and devoted representative of French interests. All the ambassadors of Henry IV had orders to make known to him the business with which they were charged and to be guided by his advice. Villeroy, the minister of foreign affairs, himself consulted him on all matters in any way connected with Rome. Ossat, through his influence and talents, secured for Henry IV the pope's aid and, when necessary, induced the Holy See to accept, at least, without public protest, such measures as the expulsion of the Jesuits, the non-publication of the Council of Trent, the Edict of Nantes, the Franco-Turkish and Franco-English alliances, the annulment of Henry IV's marriage with Margaret of Valois, and the conclusion of that between the Duc de Bar and Catherine de Bourbon, Henry's sister and a stubborn Calvinist. At the same time d'Ossat used his influence at Rome for the benefit of the historian de Thou, the philosopher Montaigne, and the savant Peirese. Clement VIII showed his esteem of Ossat by commanding that the cardinal's family should attend his obsequies with all the assistants at the pontifical throne. D'Ossat was buried in the church of St. Louis of the French, where his tomb is still to be seen. Bentivoglio, in his "Mémoires", says of him that never was a man more worthy of the hat because of his religious zeal, the integrity of his morals, and the eminence of his learning.
In the course of his diplomatic career d'Ossat wrote many letters and memoranda. Garnier de Mauleon edited some of them in 1614, when they were printed for the first time; several editions, largely augmented, afterwards appeared, the best being that of Amelot de la Haussaie, in 1708, which contains nearly 400 letters. Since then twenty-one letters have been published by Tamizey de Larroque, and eleven by the writer of this article. These letters formerly served as models for diplomats, owing not only to the importance of the questions which they treat, but especially to the talent for exposition which d'Ossat displays in them. The French Academy inscribed Ossat among the "dead authors who have written our French language most purely". Wiquefort in his "Mémoires sur les ambassadeurs" finds in them "the clearest and most enlightened judgment ever displayed by any minister", and Lord Chesterfield wrote to his son that the "simplicity and clearness of Cardinal d'Ossat's letters show how business letters should be written". Besides these letters his published works are: "Arnaldi Ossati in disputationem Jacobi Carpentarii de methodo" (4o, Paris, 1564) and "Arnaldi Ossati additio ad expositionem de methodo" (Paris, 1564).
D'ARCONVILLE, Vie du cardinal d'Ossat (Paris, 1771); DEGERT, Le cardinal d'Ossat, eveque de Rennes et de Bayeux (1537-1604) (Paris, 1894).