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Nope, it's just insulation then MDF. Where is the vapour coming from though? There's a vapour barrier externally, is that sufficient?
nick_warner-
You need a vapour barrier on the warm (inner) side of the insulation, and a vapour permeable membrane on the cold (outer) side of the construction - your building paper wrap. The foil on the insulation is a vapour barrier, but you should tape up all the gaps as well to make a continuous barrier on the warm side.
The vapour comes from inside - the inside of the room is relatively warm, you're breathing, the air is holding that as water vapour, and there's a sort of low level air pressure gradient from inside to out. The insulation means that the inner face is warm, and the outer face is cold - the bit touching the OSB. So if the relatively warm, humid air gets to the OSB, there is a risk that the water vapour condenses onto it and causes damp.
In reality, if you're not running a bath in there on a regular basis, and importantly if the building wrap really is breathable, then the OSB shouldn't get that damp and any low level dampness should evaporate out the other side. Assuming there is some kind of air-flow between building wrap and roof cladding.
hoefla
Is the ceiling vapour sealed behind the MDF? I can see a load of foil tape behind the battens. You want to avoid condensation on the OSB.