I wouldn't worry about the Blood Oxygen sensor. Its just a party trick and it isn't giving any actual useful data.
It’s not really a party trick. It actually does provide your blood oxidation level which is useful if you have Covid. My friend had Covid and was really really sick in early March, and one of the first things doctor told her to do was to monitor her blood oxygen levels. If you look at Amazon the sales of blood oxygen monitors was through the roof. I think it’s a useful feature to have especially given the status of the current pandemic. One of the signs of upper respiratory distress is your blood oxygen levels. Obviously they don’t diagnose Covid, but doctors do urge patients who have contracted it to monitor these levels. That is one of the reasons I upgraded to Series 6. I’m very high risk, immune suppressed, 2x breast cancer survivor. I like the idea that my watch has this capability. So for me, it’s not a party trick but a useful tool for my health. An acceptable range is 95-100%. Next time I have to see my doctor I will compare my reading to theirs to see how accurate the watch is since they take these levels on every visit.