Never use OSB, chipboard, or any type of particle board, as the nails will loosen and floors will squeak. I did not know this and this is what my contractor used when putting down my floor. How can I fix this? I hope there is an answer?
I can only make the following suggestions if you have not yet installed your hardwood floor over the OSB.
At the very least you should nail down 5/8″ plywood (over the OSB) hitting the joists with 4″ spiral nails. I say this is the least because the hardwood flooring nails will still penetrate the OSB 1/4″. But I would hope this would not cause too many squeaks in the future. And this will keep from raising the final floor level too much, so as making too high a rise to the various stair cases.
But 3/4″ plywood would be better, as the flooring nails will almost only contact the plywood, and be as if the OSB wasn’t there at all.
And the very best thing to do is to remove all or most of this substandard building material (OSB) and replace it with 3/4″ plywood, or even 1 by 4 Southern Yellow Pine laid diagonally to the joists. This last suggestion makes the very best nailing surface for hardwood flooring, if you’ve got really deep pockets.
But if you have already installed the hardwood over the OSB, be sure to keep your house relative humidity within 40-60 %, and hope for the best. If you find squeaks develop before your new home warranty is up, but sure to complain early and often to the builder.