You can measure your blood oxygen level inside the band by pressing the button six times until you reach this screen, press the button for 4 seconds and wait until your SpO2 (blood oxygen level) appears on the screen.
You can also measure this from the app by navigating to "Monitor" →"Health".
You'll find here all the history for this metric as well.
NOTE
*there are a lot of medical factors that can cause the readings to not be acurate (low red blood cells, anemia, various blood diseases