I suggest that those claiming that thermal runaway in Li-ion batteries produce oxygen in the off gasses would document the source of that claim.
It produces oxygen but not necessarily in the vent gases...
http://cdn.intechopen.com/pdfs/10407..._batteries.pdf
Thermo-chemical process associated with lithium cobalt oxide cathode in lithium ion batteries
snip
Flow of current over and above the tolerable/standard charged state causes instability to the cathode which starts to release oxygen into the electrolyte. Thus released oxygen reacts exothermally with the lithium plated over the graphite anode and increases the temperature of the cell making the cathode to release oxygen further. The cell which was in the dormant state in the absence of oxygen and heat now becomes an explosive device in the event the cell is met with any abuse.