When you say too much oxygen, I suspect you are meaning, too much air or ventilation (based on our PM discussion).
Some people on xpap therapy (xpap therapy meaning general cpap therapy) they can also be on supplemental oxygen which is added into the air coming from the cpap unit. Supplemental oxygen is usually for people with lung problems or recovering from surgery etc:. The oxygen is produced from an oxygen concentratoe machine or from oxygen bottles.
What the headaches could be is over ventilation which is when the machine is trying to pump more air into someone than they need. But that tends to keep the person awake rather than give headaches. Headaches are typical of a mask being too tight or the person fighting discomfort from the straps / mask / hose.
Because hubby was titrated at 16, I doubt that you setting ipap to 15 is the problem. If anything 16 is what I would set ipap to but would keep the epap to no less than 4 or 3 below ipap.
Does hubby have any lung problems (I gather from your PM that he is probably in general good health apart from the OSA).
Good luck & keep asking