One time you said to install 5/8 inch plywood over 19/32 OSB (which I have).
Another time you said to install 3/4 inch plywood over OSB. A third time you said to remove the OSB. So which is correct?
All three answers are correct, it just depends on your budget and other factors. 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 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 floor 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. I will always try to answer each question with first, what is most practical, and last, what is the most ideal, but I cannot take too long on each question.
Now, could you please answer me these two questions? Did you really ask your question about the OSB three separate times, and what do you intend to do about this subfloor?