I have ripped up 800 sqf of old redwood hardwood floors. When I did I noticed the subfloor was 1x6's at a 45 degree angle. Some were cupped and had spaces between them to the subfloor below (ground). I covered over the whole floor with 1/4 osb board to level it and cover over the holes. I put down 15pound tar paper over the 1x6's and then the osb over that. I am going to put down brazillian cherry over that, my question is should I use staples or nails. And how far will 2" nails go into the 1x6's? I just want to make sure that the nails will go into the 1x6 far enough to get a good hold. Any thoughts? ps I also put down paper over the osb. Thanks

It matters not. Staples versus cleats into lumber is a dead contest. Neither holds better than the other.

I prefer staples. They pull your stuff tighter. 1/2 in crown does more work than a 1/16th ledge.

So far, so good. Just make sure that everything is solid and secure before you nail down the b. cherry. I would have re-nailed and/or screwed down those 1x6's prior to installing the 1/4" OSB but if they are solid, no problem. Whether you use cleats or staples really doesn't matter. Both are acceptable. Don't bother to attempt to hit the floor joists, you won't, because at the angle the fastener enters into the substrate (approx. 45 degrees), nothing will be protruding past that 1x6 subfloor. I'd cover the ground below with a 6 mil polyethylene vapor retarder to ensure against moisture vapor intrusion into the subfloors.