I have not used the Ohishi, but must ask why you believe it important to have more grits than available from, say, Shapton?
For most waterstones, one needs three grits - coarse (around 1000), medium (around 5-6000), and fine (8-13000).
One can even jump grits, such as I start with a medium stone when the blade is freshly hollow ground. Or, sometimes I just re-fresh the edge on green compound (rubbed on hardwood, not leather).
Regards from Perth
Derek