Is There Free Variation Between S and Ooh?

There's apparent free variation between S and Ooh, the high back unrounded vowel in certain environments. I have a feeling that if we knew more about, uh, japonic sort of historical linguistics, we could try to figure out where that comes from. And there might be some obvious historical route to it, or there may not,. they may not know enough about it to even know that way. But it happens, it happened. So we don't know how it happened, but it happened.