Subfloor prep for hardwood install
I'm not an expert, but here is what I found out when I redone my bathroom.

The OSB is used to add stability to the floor. When walking around, if you're only on plywood, the floor will bounce; simply adding the hardwood floor over the plywood sub-floor will not solve that issue.

When installing the OSB, using floor screws is sufficient. No nails, as these will move and you'll end up hearing them. And while you're at it, you should screw the plywood to the joists if it's not already done.

And if you could get tongue and groove OSB, you'd reduce even more the risk of hearing your floor ;)