Rough-sawn boards or OSB panels for the attic (possible later expansion)

Hello everyone,

Recently, our shell construction was completed. In the meantime, I would like to make the attic floor walkable.
Now, there are two different opinions on which material is best suited for this.

OSB or tongue and groove boards?
We will later have the following structure (see attachment).
The vapor barrier (blue) will be installed on the upper floor on the wooden beam ceiling from below. The intermediate ceiling will be insulated (green).
I would now like to apply a covering (brown) on the rafters.
OSB is vapor-tight, whereas tongue and groove boards are not.

The attic is primarily intended to be used as storage space.
For this purpose, I would choose tongue and groove boards.

What needs to be changed/considered if I decide in x years to convert the attic into living/office space?
New insulation on the inside of the roof, extending the vapor barrier up to the ridge and bonding it with the existing barrier, etc.?

Many thanks for your assessments
Regards
Julian

Install crossbars and tongue-and-groove boards. OSB can be done if everything is 100% sealed. But I don't think so. Everything you don't do yourself and so on.

Later office:

The floor structure must be reconsidered because of impact sound insulation, etc. Then insulation between the joists, vapor barrier/vapor retarders. Then as desired.
Remember the following things: Have heating, electricity, water, sewage prepared!
Stick to the system for heating. So don't suddenly use radiators if the rest has underfloor heating. Basically plan everything as if you are finishing it right away.

Is it even reasonable to convert the attic? How much width over 2m do you have there? With floor construction and roof insulation, there isn't much left.

If you insulate the attic space completely right away, then OSB on the floor is not an issue because it never gets really cold up there, so the OSB boards do not get cold and no condensation can form on their underside. If you want to keep it as a cold roof, then rather use tongue and groove boards.

We have now converted the attic space in its raw state during the construction phase; insulation wool as insulation between the rafters does not cost a lot. Rafters are also clad with OSB from underneath, as (if tongue and groove are glued) this is acceptable as a vapor barrier. Floor also OSB (22mm) directly on the beam structure. The effort (labor and costs) was manageable and now everything is sealed, for example also against pests. In summer and winter it serves as additional thermal insulation for the ceiling of the living floor below.

It really looks in the photo as if there is no space to use it somewhat reasonably as living space. On the topic: floorboards are the better choice, best laid with a 2-3 mm gap. That should best help prevent damage.

[Rafters also clad from below with OSB, since (if tongue and groove are glued) this is permissible as a vapor barrier.]
Can you simply wallpaper over it later?