Always interesting to see after-the-fact discussions on the "art" involved.
O'Sullivan was, in the words from the above Daily Mail link, an "Irish tough guy" who was simply hired to provide factual documentation for the US Government's Geological Exploration of the Fortieth Parallel project, from western Nevada to eastern Wyoming. The primary goal was to attract new settlement to the American West. And as we all know, that goal was acheived.
While he may have been adept at deciding where to set down a tripod—and the resulting photographs are indeed fascinating documents—I'm not so sure the "artiness" of his task was uppermost in his mind at the time. These were hardy men on a very difficult mission. And no aspect of it more difficult than his, given the state of photographic technology at that time. If anything, his work is closer to the US Government's Depression era FSA project than to anything AA did years later.
It's not always about High Art.