Thanks for the replies. I will talk to the contractor tomorrow. What
they put down in my house is regular "APA rated sheathing", not sanded,
no tongue and groove. I specifically excluded OSB from all other areas
of construction (wall and roof sheathing, bathroom subfloor), but did
not do this for the carpet underlayment since I had never even seen OSB
rated "underlayment", so am a little frustrated that they went cheap on
me there. Just tells me that one should never "assume" anything in a
contract...
Regards, Wolfgang