Installing 3/4 inch tongue and groove oak hardwood on osb 3/4 inch thick. Can a 18 gauage 3/4 inch fastener be used for install or is it not considered proper, or does it very with manufacturer? Would like to use it along with my 16 gauge to do more but want to be correct with specs. Any info appreciated. Thanks, Steve

15 gauge 2 inch is usually what's called for. Lighter gauge would need glue too. Manufacturer's specs?

You can use an 18 gauge. I use an 18 gauge on many harder exotics.
You need a longer nail than 3/4 however.
Also I am assuming you are talking about a nailer that shoots a cleat not a stapler.
I would not mix the 2 cleats I would use one or the other but not both.