Omachi rice can be and often is grown outside of Okayama. But Okayama is its birthplace, and without a doubt the best source for premium Omachi rice.
Omachi was originally discovered in 1866, in a village of the same name in the western part of Okayama Prefecture, where almost all Omachi is grown. It is, for what it is worth, the oldest pure (i.e. not a product of cross-breeding) rice variety in Japan, and was one of the three most widely grown varieties in Japan during the Meiji period (1868-1912). Back then it was also popular as a table rice.
But its suitability to brewing great sake soon became obviously apparent, and more and more brewers in western Japan began to use it. In fact, once upon a time, it was almost common sense that Omachi should be used when brewing top grade sake for contests and such. This was before the days of Yamada Nishiki, and other crossbreeds.
They tend to be much more earthy and decidedly herbal than fruity and flowery. Aromas are in general less prominent than they might be with sake made with, say, Yamada Nishiki. The individual flavor components compete against each other in a healthy way, as opposed to blending harmoniously, as they might with Yamada Nishiki.