Ouka is at once creamy and bity, resulting in a superb feedback. It ain't hard. Morihei 4K polishes a good deal higher, and is quite harder, but creamy also defines it rather well, and on the softer side of 4K stones in general. On the harder side of creamy, NP3K comes to mind, but not like the other two - more like slightly glassy and creamy but it's freaking good a feeling for edges it leaves. Except the Ouka - and even it to some extent, but still showing quite a switch in muddiness - all these stones polish more on the mirror-prog kind of thing than hazy, although they can all be used to good effect that way with a more seasoned approach - possibly I'm sounding negative because such care is not how I like things to go, so I eventually mostly discarded working polishes up that way. Ouka is usually as far as I'll go, except perhaps on mono carbon - Morihei 4K not being exactly easy but still straightforard enough and pretty rewarding there. Even so, I'd rather use my old polishing prog and a few dirty tricks to produce similar results. It's like 10-100x faster depending on knives.
Notice that I've kept within the line of synthetic. Which is really all I know.