Thanks for starting this oko thread! I'm lucky enough to be married to a Hiroshima native, so about once a month, I get okonomiyaki at home. Most of what's said above seems to jibe with my experience, but my wife sums up the difference between the styles like this: In Osaka, the main ingredient is flour; it's a real pancake dish. In Hiroshima, the main ingredient is fresh shredded cabbage. Of course there's tons of variation no matter where you go, but the Hiroshima natives don't usually spend a lot of time adding extra ingredients like seafood, etc. Soba & pork, maybe some extra green onions, Otafuku-brand oko sauce (made in Hiroshima, available here at Japanese supermarkets) and bonito shavings. Of course, if you're ever in Hiroshima, you have to check out the oko building. Four stories of nothing but oko shops!