What's minimum safe O2 level?

In general for a patient in a hospital we try to keep the O2 sat at or above
92%, and will give supplemental oxygen if it falls below.

On a recent trip at 10,000 feet I felt bad after around three hours, and
checked my sat, which was in the low 80's. My eight year old son complained
of a headache and nausea, and after putting on the oxygen we both felt a lot
better as our sats returned to the 90's.

I had recently undergone refresher training for the Air Force in the signs
and symptoms of hypoxia in the altitude chamber, and for anyone flying at
altitude I would strongly recommend a chamber ride. The symptoms of hypoxia
can be subtle, and can vary from individual to individual.