Attic expansion with OSB and vapor barrier

Hello,
I built over a year ago or rather we moved in.
The upper floor was built with roof battens, drywall panels, a vapor barrier, and insulation above it and was approved by an inspector. The insulation and also tests beneath are still positive, everything is completely dry.
Now I want to screw OSB boards, which have been stored in the attic for a year, over the insulation onto the beams.
However, I am a bit unsure about mold growth. Actually, everything is dry and the vapor barrier is properly attached.
A wood moisture meter now showed that the OSB boards have a moisture content of 5-10%, which according to the device is low.
However, I have concerns about mold.
Should it still work since the boards will probably never be 100% dry and the moisture, if at all, will rise upwards? On the outside, I would leave about 2-3 cm space all around so the boards can expand. However, it would then not be completely “sealed.”

Is this procedure harmless?

The description of the construction sounds strange. So, from the inside to the outside, you have the following structure:

Insulation (how thick?)
Vapor barrier
Plasterboard

?

he means the ceiling, so you have to think from bottom to top. you can safely install the OSB boards there, the structure fits, it will never be 100% dry, that would also be unpleasant.

Ok, that could of course be the case, the term "Dachlattungen" confused me.

When it comes to the top floor ceiling, I would make sure that the OSB boards are not fully "sealed" over the entire surface, because otherwise you create an airtight sandwich from which any moisture that has penetrated cannot escape. Regarding the residual moisture in the boards (or also the installed KVH), you really don't need to worry about that, it's more about leaks in the vapor barrier, which usually creep in during installation.

So leave about 20cm or more free at the edges and ideally also include a small gap between the boards. Alternatively, some drill holes in the boards. There should be air circulation possible in every single rafter cavity below the OSB.

Wood is considered dry from 20%... So you can install it without worries.

Thank you very much, the answers have already helped me! One question though. If I leave space on the outside and the attic is not completely sealed, then moisture can get under the OSB boards in summer, right?