I tried blood oxygen and the readings were the same as my pulse oximeter (though it always shows 98%, so I haven't managed to test any other value), but my sleep reporting with the ring would regularly be three or four hours longer than I actually slept, making it useless.
Hyperventilate for a minute or two. Then, make a full exhale and hold it. You should be able to hold your breath for longer than you normally can and during this time you should see the value drop a bit. Be sure to inhale before you start getting dizzy or faint. (Note: do not do this under water)
