Thickness of subfloor

Q: I had a two layer subfloor over a large, well vented crawl space. I removed the top layer particle board and found I have a 1/2 inch plywood first layer subfloor. If I screw down this first layer, would my new 3/4 solid oak floor be strong enough on top of it without putting a second plywood subfloor layer down?

If I should need the second sub layer, how thick of plywood on top of the existing 1/2 inch?

A: Interesting question. 1/2″ plywood is strong enough IF when you install the 3/4 flooring, the ends of the boards actually stop on the middle of a floor joist and not between the joists. That is a pain in the neck. If it were my place, I would screw down minimum 3/8 spruce sheeting first.