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He enlisted in September 1861 in the 1st New York Engineers, and took part in the battles of Port Royal, Fort Pulaski, James Island and others in the Department of the South until December 1863 when he was discharged as a sergeant-major and brevet second lieutenant.
When the Civil War began in early 1861, he was commissioned a first lieutenant in the Union Army (Joel Snyder, O'Sullivan's biographer could find no proof of this claim in Army records) and, over the next year, was present at Beaufort, Port Royal, Fort Walker, and Fort Pulaski.
In an effort to save the old fort, the War Department finally declared Fort Pulaski a National Monument on October 15, 1924 by presidential proclamation of Calvin Coolidge.