I took my blood oxygen on the Apple Watch 3 times yesterday morning, one straight after the other and I was surprised to see utter randomness in the results.
It made up my mind that the new blood oxygen feature on the Apple Watch is useless, but I was completely wrong.
My wife explained to me that she often sees readings of around 90 and will then ask the patient to take 2 deep breaths. She waits a minute and boom, 99 will appear.
Apparently it is perfectly normal for your blood oxygen stats to change throughout the day and to change quite a lot within seconds so there is no need to panic unless it stays low for some time.
This does make it difficult to judge the Apple Watch approach, but in my tests it does appear to coincide with what I get from a standalone monitor. And I must say that the number of comments and reviews I have read that criticise the feature for random results completely miss the mark.