Start Workout on Apple Watch.
Start a Workout using the native Apple Workout app (3rd party apps like the Nike Run club app or Strava won’t calculate V02 max for you)
Choose the Right Workout Type. You will have to choose from the following Workouts: Outdoor Run, Outdoor Walk, and Hiking (you’ll notice these are the workouts which use outdoor GPS… this means Indoor Run won’t work)
Workout for 20 Minutes or More. Do the above workouts for at least 20 minutes straight without pause
Check the Health App. After you end the Workout open the Health app and you should see VO2 max as a new data point.
VO2 max relates to maximal exertion. So you’ll need to log a pretty intensive workout to generate a reading. And you’ll need to sustain it for at least 20 minutes.