Osaka has long been a great center of commerce and activity, but likely doesn't stand out as a major brewing center in the minds of most people. True, it has never been nearly as significant as its Kansai cousins -- Kyoto, Hyogo and Nara -- but the sake brewing culture was, and still is, strong there.
Osaka has historically been blessed with clean water and good rice. Things today are certainly not what they were hundreds of years ago, for either water or rice. But long ago water in Osaka was good all around, and tiny breweries existed (either officially or otherwise) in abundance, especially in Kawachi, Ikeda and Izumi. When Toyotomi Hideyoshi built Osaka Castle, Osaka consumerism boomed as it grew into a true castle town. Naturally enough, so did the demand for sake. Sake production in those three areas took off. At one point there were 38 sakagura (breweries) in Ikeda alone.
Hideyoshi was known to be fond of a sake called Amano-zake, brewed in a temple named Kangoji on Mount Amano. It was (and still is) brewed using koji that is much further along in its starch-to-sugar converting than koji used in normal sake. Amanozake is darker, mustier, sweeter and more tart than modern sake. One Osaka sake, which uses the brand name Amanozake, re-creates the original style in a sake they call their Amanozake Boso-shu.