Hello to everyone, and sorry in advance for bringing up this well discussed topic. I can't find a definite answer, so I summon your opinions here.

I’m doing my first install of 3/4 in solid unfinish hickory over 3/4 OSB board. This might not matter but I will be using 3-4-5 in wide planks. Can anyone comment on the fasteners:

1) I was planning to use a 16 gauge 2 in long L cleats with a floor gun
2) For the first row: I already own a 16-gauge finish nailer. Should I buy a 15 gage instad (expense is not an issue I just want to do it right)
2-B) On either one gauge: what length should I use (1.5in? maybe pre-glued)

THX very much in advance.

The difference between 15 and 16 will not make a difference in holding power. You will also need to blind nail at least one row of flooring at the start. If using a finish nailer it would be better to use a 60 degree angle instead of trying to get a 45 degree angle to fasten the board down. Cement coated nails will hold much better than bright finish nails. Some nails have a coating from the collating process and others don't.
I use sub-floor adhesive to help hold the starter row. The nails hold the planks in place until the glue sets. Sub-floor adhesive stays flexible and has a little stretch.
Longer nails will hold better in OSB, in my opinion.

Thank you SO very much.
I have 2-3 wks of acclimatization time to do more due diligence. I like the idea of the subfloor glue. Would I then not put the paper underlayment for the first few rows? Ill research that as well.

Also, and this might be a whole different topic, yesterday after I pulled the carpet and padding I realized it is much more noisy (new install will be on the second floor above kitchen) So I'm now considering 3mil cork underlayment at the risk of some possible squeakiness later on. With that in mind if I still want to use the glue on the first few rows would I spread: glue - cork - glue -wood-floor?

First rows would not have paper under them, as well as the last rows.
You may want to glue down the cork with the glue that has little chunks of rubber in it to help reduce sound transfer. The fasteners that penetrate the sub-floor will help transfer sound waves. Two layers of glue with the cork in the middle and flooring on top of the cork would be the best against sound transfer.

"Two layers of glue with the cork in the middle and flooring on top of the cork would be the best against sound transfer."

Let's make sure I understand it correctly: would I still top nail/blind nail the first row and cleats the rest over the top layer of glue? So from the bottom up: glue/cork/glue/nailed flooring. Thx

What about Quiet Walk Plus. The plus is intended for nail down solid hardwood.
or
Roberts Super Felt?