I cannot found the blood oxygen sensor app in my watch

You need to have the Series 6 watch and be running OS7

You cannot be running beta OS or IOS

You have to be older than 18

See https://support.apple.com/en-ca/HT211027

I’m guessing you have the Apple Watch Series 6. If you do, and you don’t have the app go to the App Store, search “Blood Oxygen” click the one that looks like the image I attached, and see if you can install it. If you have a series 5 or lower, blood oxygen is not supported.