Insulating wooden beams of the attic ceiling

So above is then fully heated and also fully used?

So it's fully heated and fully used at the top then?

Yes, that's exactly how it is.

Then it is more about soundproofing than insulation, since the above also counts towards the thermal envelope. That is easily doable without having to worry about mold. But I don't have much knowledge about that and prefer to refrain.

Why is it then more about soundproofing? He wants to insulate the upper floor from the attic. In any case, I would not use OSB boards as the top layer because they are also used as vapor barriers. That means it is sealed upwards, where it should be more diffusion-open. Less ideal, so better use tongue and groove boards with a little space between the individual boards. Honestly, I wouldn't know why it should work without a vapor barrier. But I am not an expert either

I have now assumed it is the intermediate ceiling?

As the top floor ceiling, okay... But then the top layer does not matter either.
I would probably tend towards cellulose there.

Okay, now I'm not sure anymore either. Of course, I am referring to the top floor ceiling.