1. Measuring blood oxygen at the wrist is useless, it's inaccurate, you measure it at finger tips
2. The apple watch (and the Samsung Galaxy Watch 3 for that matter) do not measure it very accurately
3. There is no point at measuring it consistently/random times during the day, rather get a proper oximeter, they're cheap enough compared to an Apple Watch (R300 for cheap and around R1k for middle going up to R2k or so for the higher end home use ones)
This is on a Watch 3:
So I got 96%, now what does that tell me? Nothing, because if you have blood oxygen issues you are probably already feeling symptoms, so no point measuring randomly through the day.
Even generally measuring HR is kind of useless, I only do it for Vitality, I can feel if I am pushing myself doing exercise already, but there it's a little bit of a better argument if you want to see how far you're pushing yourself while the blood oxygen test tells you nothing.