9mm reloads
 

I loaded up some 9mm using 4.8 grains of Unique over 124 grain Hornady JHP. When checking the powder load in the brass I noticed there was not much space for seating the bullet and that the powder would probably be crushed. Is this OK? Brass was measured OK for lenthg.

What is your OAL?

Quickload says 87% filling at max. (1.169") so you should be good.
Even if you seat to 1.13" there should be a little air.

Some powder occupies more volume by weight than other milled shapes.

Titegroup is known for being compact. The downside is that it's easier to double or overcharge a case with a compact powder.

Would a "full" cartridge case become oxygen starved? Unlikely. There is a fair amount of oxygen bound up within the powder granules themselves (in addition to carbon, hydrogen) of nitrocellulose. There is also oxygen between the grains of powder, which never sit solidly against each other. The surface of powder is also porous.

https://en.wikipedia.org/wiki/Smokeless_powder

There is some interesting discussion about the way powder within a cartridge case burns. Taken to extremes, it becomes more important in artillery and canon design.

Unique is a flake powder. It has a low bulk density.
It is NOT going to be 'crushed' by the bullet pressing down on the charge column.
Compressed loads are NOT a problem any way, other than in some folk's minds.