William just called and I feel like a fool, but a grateful fool. The green "tree" on the concentrator (where the O2 hooks in) was really loose and caused most of the oxygen to be lost before getting to the ventilator. So it was probably all mechanical, but a simple fix - tighten the stupid connection. I've gotten used to tracing his oxygen line when he struggles to make sure it's all plugged in, but never thought about the connection. One more thing to put into the checklist.
But some of his labs are a bit squirrley so they're going to watch over night. They want to make sure the skewed values are related to the stress of trying to get enough oxygen last night and not something else brewing.