O'Sullivan made his photographs a strong documentation of the war by depicting it throughout his work from different perspectives, and thus thoroughly and creatively. He also used visual perspective and composition to make his photographs strong individually, expressing the war originally and powerfully within the limits of each frame.
Having identified O'Sullivan's challenge as having to document an entire war, the first thing we might notice about his photographs is the range of subject matter. He photographed the human side of war with "individual and group portraits of nearly every type of man involved, including officers, enlisted men, and hired civilians."1 Photographs of "almost every kind of military agency and station, from post offices and repair shops to mess halls,"2 depicted the incorporation that the war catalyzed in the North. He recorded the technological feats and innovations of the war in photographs of battleships and pontoon bridges being built by engineers while under the fire of sharpshooters.3 And he portrayed the towns, buildings, and empty fields to which the war had given historical significance in one way or another. It is this last type of photograph that we most struggle with today, the banality of the images demanding a rigor of imagination uncalled for in our modern culture of virtual realities. The range of O'Sullivan's subject matter is analogous to his first-hand experience of the war, and bespeaks an interest in portraying it as comprehensively and accurately as possible. He perceives and portrays its myriad reality through multiple perspectives.
O'Sullivan discovered that the war could also be viewed and portrayed powerfully from the perspective of its aftermath. If views of military camps and soldiers portrayed the structure, comradery, and boredom of camp life, and views of ships and bridges portrayed the technology of war, views of the changes wrought on the landscape, from the remnants of forts and earthworks to ruins of buildings and bridges (what Timothy Sweet calls "traces of war,"4) portrayed the violence and impact of the war in the most physical terms. No doubt, part of the appeal of these views was that they were accessible to the photographer. But they were also powerful images that brought home the reality of war as destruction.