Northern Virginia Campaign
After being honorably discharged, he rejoined Brady's team. In July 1862, O'Sullivan followed Maj. Gen. John Pope's Northern Virginia Campaign. By joining Gardner's studio, he had his forty-four photographs published in the first Civil War photographs collection, Gardner's Photographic Sketch Book of the War. In July 1863, he created his most famous photograph, "The Harvest of Death," depicting dead soldiers from the Battle of Gettysburg.