I recently set up one of my activities to track HR and O2 on the data screen. The O2 fields kept coming up with double dash. So, I sent in a tech support to Garmin.
The first rep told me that there was something wrong with the Garmin Apps.
I then got a 2nd rep who told me:
The ability to analytically extract respiration rate from HRV data depends on accurate heartbeat data. The degree to which one heart rate changes from one beat to the next can be a matter of milliseconds. Challenging monitoring conditions related to the performance of physical activity mean that the pinpoint accuracy needed for this analysis is only achievable using a chest-belt type heart rate monitor
So, the watch can't do o2 and HR at the same time and you're supposed to wear a HR strap to cover the function. I really wish Garmin had put that in their sales material as I would have never bought this watch.
