If you are using Samsung Health on the Note 8, the oxygen saturation is now located under the stress option inside of Samsung Health.
Unfortunately when they moved the 02 sensor functionality completely under the stress monitor as opposed to having it as a separate function, they disabled logging of the 02 data.
I recently downloaded my data files in an effort to track 02 levels given the current Covid-19 outbreak and found that the last entry in the 02 data file was June 25, 2019 - about the time the change was made to the software. I've looked through the other data files (e.g., stress) and found that the 02 data is not recorded in any other location. So while you can see your 02 reading under the stress function, the 02 data apparently isn't logged anywhere and cannot be retrieved at a later date. If someone does find it is logged elsewhere after the change, I'd be happy to hear but I have been unable to find it.
because samsung recently is drafting away from what people expect from the phones. The solution is to uninstall the latest update of samsung health and the SpO2 feature will be there. It works on my S7.
I now see that my "Measure Multiple" option was on by default, so I've always had my oxygen saturation displayed. It's the menu item next to the Share option on the upper right side of the Stress screen. Hopefully you have the menu - Open that menu to turn it on.