There certainly appears to have been an American racer called Robert O'Brien who appeared briefly in SCCA events in 1951 and 1952 racing his own Jaguar XK120. Bob hired a Ferrari 166MM to take 4th place in the first Sebring 12 Hour race and also drove a Connaught at Bridgehampton where he was photographed, a picture that confirms that the gentleman (shown left) observing Bira at the 1952 Spa race is indeed the mysterious Mr O'Brien.
It's strange that more is not known about O'Brien. He seems to have been a run of the mill SCCA racer and one would have thought that there would be many people from that era who would remember the man. There is also some confusion with Bill O'Brien, a Frazer-Nash owner, who entered but did not compete in the 1952 Targa.
A final mystery, on the last day of August 1952, O'Brien competed in the Grenzlandringrennen F2 race in a Simca-Gordini entered by the Viscomtesse de Walckiers, a lady who does not seem to exist except on motoring web-sites.