Omachi rice is the ancestor of most of the modern rice used for making sake. It's famous siblings are Yamadanishiki and Gohyakumangoku. It originated around the prefecture of Okoyama and the people of Okoyama (and Katsuyama where Gozenshu sake comes from) are justifiably proud of it.
The rice has a very nice 'taste' but unfortunately is getting more rare because it is hard to grow. If you look at the picture above (taken by myself in the showroom at Tsuji Honten (Gozenshu) shuzo) you can see the difference in height between normal and Omachi rice. It's taller but unfortunately this means it's more susceptible to high winds and hail etc so can be damaged easily.
That's why you can find a lot of sake much cheaper. The rice isn't as good. (We're not talking about cheap 'futsushu' sake where they add 'jozo' alcohol (a bit like 'metho') to beef up the alcohol content but is only able to be drank warm or hot. We don't generally do those here at SuperSake).
If you're after sake with Omachi rice at SuperSake you would want to taste sake from Gozenshu namely the Kei, Mimasaka and the 'Gozenshu 9' (made with the ancient Bodaimoto method). Also our 'off list' nama genshu muroka sake (Sumiya Yahei) but as it is a 'yama' sake this can only be purchased from our 'liquor store' located at Tsunami Mosman Park, (18 Glyde street). The reason being is that it has to be stored at 5°C so we can't send it in the mail. Gomensasai :-(