Maybe I didn't make the picture clear enough: I don't keep breathing throughout the set, I just learned to breath at different "landmarks".
I used take a big breath and keep the air in from squatting down and back up to lock-out. I would then take the needed breaths for the next rep.
And I don't thinks its an oxygen issue, I think its blood pressure. Not enough oxygen would cause you to pass out.
What I do now is this:
I fill my lungs, squat down and as I rise I exhale. Very controlled. This is about 2/3 of the up movement. To get out the hole you want to the tension in your upper body or else you would simply collapse on heavy weights.
Maybe you wanna try that with a light weight, as your warm up sets to get used to that new pattern. Do it very consciously and feel the point where to exhale. I know that sounds sloppy. Try to use the pain as a gauge.
When I felt it build up again I started to let the air out.
Thats what helped me.