Many of the above responses confuse me, but I am aged and a little slow. Anyways . . .
In primary fermentation, a little headspace is fine, you'll need it for krausen and the space will fill with the CO2 the yeast are pumping out, so your risk of oxidation is low.
If you are transferring to a secondary fermentor, then headspace and oxidation is more of an issue. You can shoot a little CO2 into the headspace after you rack, if you are concerned.