When the
American Revolutionary War broke out, in 1775, De Lancey was at once despatched to his native colony to make arrangements for the accommodation and remounting of his own regiment and of the royal artillery, then under orders for active service. He found on his arrival there that his father had warmly espoused the royalist cause, and in the following year the elder Oliver de Lancey raised and equipped at his own expense three battalions of loyalist Americans, which he commanded with the rank of brigadier-general. The younger Oliver de Lancey accompanied his regiment to
Nova Scotia, to
Staten Island in June 1776, and then on an expedition to
Long Island, where he commanded the cavalry outposts in the smart action of 28 August, in which the American General
Nathaniel Woodhull surrendered to him. It is agreed that Woodhull was assaulted after he had surrendered and died of his wounds. Some sources accuse De Lancey of encouraging his men to maltreat Woodhull and of using his sabre to wound Woodhull, while some others say that De Lancey tried to prevent the attack. De Lancey commanded the advance of the right column of the British Army under Sir
Henry Clinton and Sir
William Erskine at the
Battle of Brooklyn, served at the capture of New York and the
Battle of White Plains. De Lancey was promoted major in his regiment on 3 July 1778. With this rank he covered the retreat of Knyphausen's column in Clinton's retreat from Philadelphia, and was present at the
Battle of Monmouth Court House, and in temporary command of the 17th Dragoons, which was the only cavalry regiment in America, he commanded the outposts in front of the New York lines from the middle of 1778 to the end of 1779. De Lancey then joined the staff as deputy quartermaster-general to the force sent to
South Carolina, and after serving at the
capture of Charleston he became
aide-de-camp to
Lord Cornwallis, and eventually succeeded Major
John André as adjutant-general to the army at New York. He was promoted lieutenant-colonel of the 17th Dragoons on 3 October 1781, and retired to England with his father on the conclusion of peace and the recognition of the independence of the United States of America. ==Military career in England==