Contrary to your belief but not contrary to science fact. Oxygen is actually soluble at just the right amount that is recommended for yeast, about 8ppm at 20c. The oxygen will not off gas to the headspace but instead be consumed by the malt and hop antioxidants.
It's not really belief, it's more the second law of thermodynamics. Above 8ppm, the extra oxygen will want to leave solution to achieve equilibrium. I honestly don't know how rapidly this happens but I'll speculate that 12 hours is enough time for this. But to your point, seems reasonable that a lot of that O2 would be intercepted by various reactions. Long story short, this wort probably had only ~8ppm, which, yes, is enough unless the yeast aren't very healthy and/or the wort is imperial strength, which we don't know from the OP. I guess my original point was that the extra oxygen, over and above ~8ppm, was likely not there.