At the age of 21, O'Sullivan joined a team of civil war photographers. The photos he's taken of the civil war are perhaps his most famous, a long with beautiful landscapes of on a waterfront. At one point in his career he underwent an exploration between the Rocky Mountains and the Sierra Nevada Mountains. Also, a lot of his works come from Shoshone Falls in which he compares to the popular Niagara Falls. He was a very active traveling photographer, leading an expedition visiting the Zuni and Magia pueblos and the Canyon de Chelly.
http://americanart.si.edu/collections/search/artist/?id=3600
http://americanart.si.edu/collections/search/artist/?id=3600