Captain Jeremiah O’Brien (1744 – 1818) was in command of Unity when she captured HMS Margaretta in the first naval battle of the American Revolutionary War.

O'Brien and his five brothers, Gideon, John, William, Dennis and Joseph, were crewmembers of the sloop Unity when she captured Margaretta at the entrance to the harbor at Machias, Massachusetts (later Maine), 12 June 1775.

Under the command of Jeremiah O’Brien, thirty-one townsmen armed with guns, swords, axes, and pitch forks captured the British armed schooner in an hour long battle after Margaretta had threatened to bombard the town for interference with the shipment of lumber to British troops in Boston, Massachusetts. This battle is often considered the first time British colors were struck to those of the United States, even though Unity was not formally a member of the Continental Navy. The United States Merchant Marine claims Unity as its member and this incident as their beginning.

A privateer, O'Brien continued as the captain of Unity, renamed Machias Liberty, for two years.

Four ships in the United States Navy have been named USS O'Brien for him. The Liberty ship, SS Jermiah O'Brien was also named for him.