Ok that makes sense that it happens at the surface of the liquid but it has nothing to do with the headspace being filled with O2. It has to do with the interaction of the bubbles on the surface of the wort. So it is all about bubble size, even using a stone if you crank the gas you will get large bubbles that form as it reaches the surface, so the most important thing is proper flow rate. Which will depend on your set-up

Filling the headspace is only relevant in that if your headspace is full of O2, then at the liquid/gas interface you have more contact between O2 molecules and your wort. Small bubbles will have more surface area than one large one, but a given bubble rising is only going to be in contact with that volume of wort for only a few moments until it rises to the surface. The entire surface of the beer is in contact with O2 the entire time you are oxygenating, and with the bubbles creating currents within, as the surface becomes saturated it is being exchanged for less saturated wort. For commercial guys that are doing inline, it is still going to be entering a tank where the surface of the liquid is going to be picking up significant amounts of O2 as well, and the bubbles are going to be in contact with wort longer than they are in a 5 gal HB set up, so they are likely getting more bubble-->wort pick up as well.

There is also the fact that the headspace is filled with O2 (until it is filled with CO2), allowing O2 pickup until fermentation really starts moving.

~widget