Keep in mind that pressurization itself is not the goal of pressure fermenting. The goal of pressure fermentation is to control ester and fusel alcohol production through the metabolic suppression of the yeast from high CO2 concentrations. In that sense, high pressure with O2 is counter-productive, especially during the lag phase when the yeast are just getting started.
You are better off helping the yeast build their sterol reserves with dissolved O2 in the wort, and at regular pressure, via bubbling O2 (or air) into the wort. at the beginning of the fermentation. O2 in the wort can cause staling reactions, so you want it out of your fermentation as soon as possible! The yeast helps here and will consume O2 in the wort quickly (in minutes! 20 - 120 mins depending on temp). O2 in the headspace is relatively inaccessible due to the slow gas exchange that happens at the surface of your wort. So yes, O2 pressure can increase that a bit, but not at the rate that you were hoping for.
For pressure fermentation, let the CO2 from the yeast create the pressure. There's no need to waste bottled CO2 for this purpose. Indeed, I use the volumes of extra CO2 created from my pressurized fermentations to purge my corny kegs ... the best CO2 you can get!