We also know when the Civil War began in early 1861, he was commissioned a first lieutenant in the Union Army (though Joel Snyder, O'Sullivan's biographer, could find no definitive proof of this claim in Army records). There is no record of him fighting. Alexander Gardner worked as a photographer on the staff of General George B. McClellan, commander of the Army of the Potomac, and was given the honorary rank of captain. Gardner described O'Sullivan as the "Superintendent of my map and field work." Biographer James D. Horan writes that O'Sullivan was a civilian photographer attached to the Topographical Engineers. His job was to copy maps and plans, but he also took photographs on his own time. Although he later listed himself as a first lieutenant, the rank was likely honorary, like Gardner's. From November 1861 through April 1862, O'Sullivan, working for Gardner, followed Union forces to Fort Walker, Fort Beauregard, Beaufort, Hilton Head, and Fort Pulaski.