The particular kind of sauce that tops the okonomiyaki is a matter of great import. It seems that it is traditionally a kind of thick sweet dark sauce, probably of early modern Javanese origin, transmitted by the Dutch traders, along with the fighting kite (see Thomas Pynchon on ketjup in Mason & Dixon), that the Japanese serve with tonkatsu. However, in two restaurants at Japan Center in San Francisco, I have had okonomiyaki with a sauce that seems to be equal parts mayonnaise and tonkatsu sauce, swirled in an (un)appetizing spiral.
sekicho points out In Osaka, the mayo/sauce combination is much more common, so I would venture that it's the "right" way to eat okonomiyaki.