Oxygen sensor

Force restart your watch by holding in the crown and side button for 10 seconds till you see the Apple logo then release

Failing that I'd try unpairing the watch from your iPhone then pairing it back setting it up as new

Hi Degrote,

It sounds like you're having issues with the Blood Oxygen Sensor on your Apple Watch. If that's correct, you may want to reach out to Apple Support here: Get Support.

Cheers.