Apple Watch just keeps getting better — 9to5Mac recently discovered via iOS 14 code snippets that the next Apple Watch might be able to detect bloody oxygen levels.
This is a feature that can prove to be very useful — being able to know your bloody oxygen levels can help warn you of health conditions, preventing delayed detection of health problems.
On the other hand, Fitbit already has a built-in SpO2 sensor on some of its devices. You can check your estimated oxygen variation to monitor health conditions such as sleep apnea and asthma.