[Anyone have any ideas on how they measure the O2 in your blood from the skin level, i.e. w/o actually sticking a probe into a blood vessel???[/quote]

FWIW, I saw this explanation on the Outside website

The other big update, the pulse oximeter, is primarily useful for excursions into the alpine. The pulse-ox on the Fenix 5X Plus uses the same laser technology as those hospital finger clips. It shines two beams through your wrist and measures how much light reaches a detector on the other side. In the case of sports, it helps determine how your body responds to high altitude, where oxygen levels are lower. Sitting in our office, the watch told us our oxygen levels were at 100 percent. That reading dipped significantly on a run here in Santa Fe—which makes sense after five miles in 75-degree heat at 7,000-plus feet above sea level. But without comparing it to a hospital-grade sensor, we can’t say how accurate those readings were.

........The average athlete certainly doesn’t need to know their blood-oxygen saturation, but that tech has huge implications for people at the top end of mountain sports—and the maximized screen size on the 5S has huge implications for people with small wrists.

https://www.outsideonline.com/2318956/garmin-launches-fenix-5-plus-series