I'm new to this site. I would guess that the following question has been asked and answered more than once. But I'm not sure what keywords to search on for it.

I want to add an intermediate layer of 3/8" plywood over an existing 3/4" OSB subfloor prior to installing my 3/8" Brazilian Walnut T&G flooring. The purpose of the intermediate layer is to raise the hardwood flooring to being level with adjacent floors. So the stack-up from top down would be:

Hardwood flooring.
15 # roofing felt.
3/8" Plywood, the intermediate layer.
Possibly glue. See below.
3/4" existing OSB.
Floor joists ( 2x10s, 16" on center).

In order to prevent any squeaking or other movement problems, I want to attach the intermediate layer securely to the underlying OSB. The question is how best to do this.

Any problems with my stack-up?
Should I glue and screw the intermediate layer?
What spacing should I use on the screws?

Bellawood, the flooring manufacturer, suggests 18 gauge cleats for nailing the hardwood. I presume I should use cleats long enough to penetrate the hardwood, intermediate layer, and almost through the OSB. Is this correct? I'm thinking 1 3/4" cleats.

How necessary is it to flatten the top of the OSB with self-leveling compound or other means prior to adding the intermdiate layer? If, needed, what to use?

Thank you,
Jerry V.

Working in reverse order of your questions:
If the substrate is not flat your hardwood wont be either. Sand or grind all high areas of the OSB, pack low areas with a cementicious compound.i would not use this if I wasnt adding the 3/8th plywood. Also, rescrew this OSB to the joists and tighten up any loose existing fasteners.
If you are going to glue the plywood down, you COULD just use subfloor staples to hold it in place until the glue sets since it is the glue, and subsequent flooring fasteners that will be doing the work. Use a urethane based adhesive to avoid introducing unwanted moisture into the floor assembly.
18 gauge cleats only come in a maximum length of 1 3/4 inch so that is it for your choices. Is this a prefinished engineered hardwood? I am assuming so because of the 18 gauge stipulation.
Good luck with the project and dont let it overwhelm you.

Dennis,
Thanks for the responses. The flooring is solid 3/8" x 3" T&G, not engineered. Bellawood, the manufacturer, recommended either 18 or 20 gauge cleats in their installation instructions for this product.

I noticed that you mentioned that I should rescrew the OSB to the joists. Is this typical? This is the first time I've run across this suggestion. Are you recommending everywhere, or just where there are questionable areas?

Thanks again,
Jerry V.

We always refasten the subfloor prior to any work above it, simply because as an existing structure settles and matures the original fasteners tend to loosen up. Use screws spaced about 8 inches apart along each joist. Dont forget to hammer down the original fasteners, they will be standing "proud" and will interfere with the installation of the plywood underlay.
For a 3/8th engineered product, I would think that a 1 1/2 inch or even 1 1/4 fastener should be sufficient.