This isn't my first hardwood installation but it's the first time I've had to beef up what it's going on top of. So far I have pulled up gross carpet and almost grosser particle board. What is left is 1/2" plywood subfloor which I'm guessing isn't tongue and groove. It's nailed down to joists that are spaced 16" on center.

I'm installing 3/4" oak perpendicular to the joists and plan to use 2" staples. I am fairly confident I need to add another layer. 15/32 plywood? 23/32 T&G OSB?

My big question is.... how do I secure it? Everything I read seems to say something different about the underlayment. Glue it! Don't glue it. Screw into the joists. Don't screw into the joists! I'm eager to hear something I can sink my staples into.....

The National Hardwood Floor Association recommends covering your floor with half inch plywood, using 8" staple schedule for nailing the field and 6" along the edge. You can use the floor stapler for this if you take off the shoe and then straight into the plywood with the 2" staples. Use a straight-edge to check for flatness and grind high spots and build up low areas with roll roofing or 3 tab asphalt shingles before the plywood. Screws would be overkill.

I think that your floor ought to be fine with a good plywood laying hardwood directly over it if it is in good shape and you are not nailing down plank, just strip flooring.. Make sure the plywood is fastened well. Fasten the oak with 2 inch staples every 10-12" and within both ends of each piece within 3 inches. Try to fasten over the joists and in between for best results. This used to be the standard years ago. Cover the sub-floor with aqua bar underlayment paper before the oak.

Thank you for replying. Based on your reply I (finally) read through the NHFA guidelines.... and in addition to your second paragraph.... if I have a secure, clean 1/2" plywood subfloor is it overkill to add another 1/2" layer? I had it in my head from previous installations that the base had to be thicker than 1/2" but that may have been due to the joist spacing. It's been a little while since my last install....

Another layer of half inch would be prudent.

Very good. Thank you!