You can install directly over the subfloor. It must be reasonably flat and secured well to eliminate squeaks and looseness.

If you want to install a plywood layer, 3/8" minimum thickness.

Reasearch vapor retarders and decide if that is appropriate for you. Note that asphalt impregnated retarders, such as roofing felt, can lead to momentary snapping noises sometimes...the wood sticks to the underside of the flooring and then releases resulting in the snapping noise. To avoid that use a slip sheet of kraft paper or choose a vapor retarder that has a kraft paper face.

Rudy,

Does your present subfloor not squeak? We have the same thing- we removed the particle board subfloor that was nailed to the shiplap subfloor (are they both called subfloors?), and then my DH glued and screwed 1/2" plywood to the shiplap and voila we have almost zero squeaking! Our 3/4" hardwood then was nailed on top of the two sub-floors. Flooring guys used what looks like a reddish-brown wax paper (doesn't look like kraft paper if kraft paper is what I mail packages in).

My subfloor squeak. It caused by loose joint. I fixed it. Now no more squeaking.

Thank you for your input. I definitely helps.