Immediately after the Battles of Lexington and Concord Oswald joined the American army. With Benedict Arnold he participated in the Capture of Fort Ticonderoga on 10 May 1775. Oswald went with Benedict Arnold's expedition to Quebec in 1775 as one of five unattached volunteers. The other four were Aaron Burr, Matthias Ogden, Charles Porterfield, and John McGuire. On the night of 31 December 1775, Oswald took part in the Battle of Quebec. Benedict Arnold and Oswald accompanied the 25-man vanguard of Arnold's 600-strong column. He was wounded and captured in the battle. He was not exchanged until 10 January 1777 when he received a commission as lieutenant colonel in John Lamb's 2nd Continental Artillery Regiment. He performed noteworthy service at Compo Hill during the Danbury Raid. During the action, Lamb's three-gun battery made a valiant attempt to stop a British bayonet charge.