"...only ambulances would have prehospital access to O2..."

We carry O2 with us, "just in case." Got the regulator (the most expensive part) and a couple of empty bottles at a yard sale (after the death of an old person), went to a med supply store and swapped the empties for full bottles for a reasonable price. Nasal canulas are readily available and cheap. You can probably get your family doctor to write a prescription for O2 if you ask. Cheap insurance...