I've only ever shot my can in an indoor range and with 55gr AE .223 ammo. Never noticed any gas to the face with the combos I mentioned above.Keep in mind I am responding without reading ANY other replies. Such my opinion may (or may not) be redundant.
I have about 7 or 8 suppressors right now...all of which are traditional baffled designs. That said if I were to do it all over again (with current choices) I would take a lot longer look at the OSS cans when looking at a specific 5.56 setup.
It's not like I don't like other cans or I don't like mine now, but for shooting indoors or in low wind conditions you will stand a big chance of getting gassed out. Also with 5.56 you will need ear pro no matter what so chasing decibels in that caliber is not the most important thing.
Gas in the face in my opinion is a function of your ammo choices just as much (if not more) than other things like gas blocks or charging handles, or even port size. Yes, more important than gas port size. Since I reload everything I've experimented quite a bit and really gassy powders will choke out almost anyone regardless of other factors and conversely less gassy powders won't.
Most of the gas coming back to the shooter is not coming through the gas port or gas tube. It's coming straight back down the bore of the rifle. With a traditional can, and especially one that is more constricted, those will stack up gas which releases when the bolt reciprocates and creates a cloud around the shooter. This also leads me to think that the Surefire suppressed bolt carrier group might also be worth a look as it delays unlocking for a bit. That paired with the OSS and I think you will have a winner for a specific indoor setup to mitigate gas in the face.
The only other can that I know of is the Griffin Explorr that is intentionally built for semi auto guns. It has a balanced approach to gas flow. I am sure others do that too, but I just don't know which ones are like that. The Explorr gives up some DB at the muzzle but it also allows more gas to flow through thus lowering some port pop and (potentially) some gas in the face. Still though in the right conditions you will still get some gas, but a bit less than a more constrictive can built for a bolt gun. The other benefit is that can is super light weight relatively speaking.
You won't eliminate gas in the face entirely, all you can really do is mitigate it as best as you can. Some suppressors are designed from the start with that idea in mind.