This one seems to be a bit of a mish-mash, hash! My big (Shorter Oxford English Dictionary) says Omphalos means 1. a boss on a shield, 2. a stone in the themple...and 3, figuratively a central point. The 'navel' doesn't get a mention other than in words beginning Omphalo (-omphalitis, omphalocele etc) and in Omphaloid, resembling the navel. Other than that, of course, we have the word here beginning Oom...