Isn't the limitation CO2 scrubbing rather than O2 levels?
I well remember that was the problem when the two Rogers were stuck in the putty on the seabed of the Irish Sea back in the 1970s. To this day Roger Chapman doesn't like to sit down, even at business meetings. He prefers to stand up. If you've seen the inside dimensions of that sub you'll understand why!
Edited to add:
Wasn't that also the most pressing limitation on the Apollo 13 recovery too? That and the unholy cold of course.