On our order list it states that we have 19/32 OSB Floor decking in both the top and bottom floor of the home. If my math is correct that is a little over 1/2" inch thick. Is this thick enough to nail our prefinished 3/4" floors to or do we need some sort of underlayment?

NOFMA states that the minimum OSB subfloor thickness needs to be 3/4". I would recommended adding an additional 3/8" to 1/2" plywood underlayment.

Thank you so much Gary. I'll call the modular company on Monday and instruct them to install a thicker OSB subfloor. Thanks again.

Hold on a minute partner. I go along with the additional underlayment idea, but for a different reason. Modulars are assembled on-site with 2,3,4,5 or more sections and each section has a cold joint where the subfloors meet. Setting the entire house then coming back and installing an underlayment over the subfloor would tie your house together better imo. That would lessen the chance of the subfloor in each module pulling the hardwood apart with seasonal changes.

Bottom line is I would have them swap to a 3/4 T&G AdvanTech and still consider adding an underlayment to boot.