Charles O'Brien, 6th Viscount Clare

Charles O'Brien, (17 March 1699 – 9 September 1761), 6th Viscount Clare (titular 9th Earl of Thomond) was an Irish military officer in French service.

Charles O'Brien was the son of Charles O'Brien, 5th Viscount Clare and Charlotte Bulkeley. He fought for France against Spain in 1718 with the rank of colonel in the service of his father's regiment and later fought in the War of the Polish Succession in the Siege of Philippsburg in June 1734, where he was wounded. He gained the rank of Maréchal de Camp in 1735 in the service of the King's Armies. O'Brien also fought in the Battle of Dettingen in 1743 and in the Battle of Fontenoy in 1745.

O'Brien was invested as a Knight, L'Ordre du Saint-Esprit of France on 2 February 1746 at the chapel of Versailles, Île-de-France, France. He held the office of Governor of Neuf-Brisach in Alsace, and was Commander-in-Chief of the province of Languedoc. In 1757 he was made a Marshal of France. He died on 9 September 1761, aged 62, at Montpelier, France.

O'Brien married Marie Genevieve Louise Gauthier, daughter of François Gauthier Marquis de Chiffreville, in 1755 and had three children:

• Charles O'Brien, 7th Viscount Clare
• Antoinette Charlotte Marie Septimanie O'Brien b. 1758