@ all not familiar with the T-6 OBOGS war stories, and possibly the F-22 system and so forth........
Several incidents at Whiting field and other bases raised a flag for the Navy and USMC and USAF.
The problems that USAF and USMC are looking at is not low oxygen, but contamination. In the "old" days we had oxygen bottles and monitoring their health and well-being was easy. We didn't test at our mask connection for bad fumes or oxygen content or..... Hell one system I flew with had constant over pressure so I had pure oxygen unless I disconnected the nose hose. Apparently, the F-35 also has a slight over pressure, but I'll have to talk with one of the pilots( that feature was brought up during the accident investigation of a loss this past May here at Eglin).
The OBOGS was intended to reduce ground support issues and the oxygen tank weight/space/maintenance costs and effort. Sounded great, and I sit here now sucking on a home oxygen concentrator due to my poor habits for over 50 years. BFD!!! Trouble is the sucker doesn't work real good above 10,000 feet pressure altitude. The standard systems I flew with for a few decades had oxygen bottles and a regulator you could actually turn off or select emergency pressure. The OBOGS doofers do not seem to work that way.