I purchased an active watch 2 for my son yesterday and had the exact same problem! I reset the watch and it disappeared completely and so far hasn't come back.
I have tried everything! Reset the watch, uninstalled both Samsung Wear and Samsung Health only to setup and install as new (without restore) but it's not there. I purchased the watch brand new from Argos here in the UK. I read somewhere that the health app on the watch needs an update and it should appear in a day or two but i'm not very hopeful that will happen.
To date it hasn't returned on my son's active 2 watch. I think Samsung has removed it! Wear, Health and firmware are all the latest versions and nothing.
This is a big scam that Samsung sells its watches with massive blood pressure and oxygen sensor ads, but in practice there is no such thing even after years and new updates.
I am facing same issue.
I have active 2 watch and unable to add Oxygen measure widget.
Looks like wasted money on Samsung. I should have searched for Apple.
Same here on an LTE variant in Austria 😕
This needs fixing, Samsung - we waited forever for this feature and then, when it finally comes, it's broken