I have received fresh information from my friend, a former U-boat officer, concerning use of compressed oxygen.
This link contains the original information, and subsequently the new information:
http://www.subsim.com/radioroom/showthread.php?t=185993
Unfortunately, it appears that no one really knows the answer today, despite my friend's searches in Germany. However, he is clear that - under normal circumstances - it was depletion of U-boat batteries that caused the U-boats to surface, not depletion of oxygen.
Stiebler.