Oliver FitzWilliam, 1st Earl of Tyrconnell and 2nd Viscount FitzWilliam (died 11 April 1667), was an Irish nobleman.
His father was The 1st Viscount FitzWilliam (1581–1650). His mother was Margaret Plunkett, daughter of Oliver, 4th Baron Louth; through his grandmother, he was a member of the powerful Bagenal family. As a younger son he did not expect to inherit the title or estates; like many young men in his position he read law, studied at Gray's Inn, and then resolved on a military career. With the help of The 1st Duke of Ormond, he became a Colonel in the French Army and showed both courage and military skill. On the death of his eldest brother Richard, he became heir to the title.