Is the SR100 worth it over an Oura Ring?

If you mainly want heart rate and SpO2, the SR100 covers those basics. Oura and Ultrahuman add detailed sleep stages, readiness scores and a deeper app, so pick them if that matters.

It records heart rate and SpO2 around the clock, which gives sleep-related data. It does not offer the full sleep staging and recovery scoring of premium rings.

Yes. IP68 means it survives sweat, rain and swimming. Rinse it after salt water.

It charges wirelessly on its dock, with no exposed contacts. A full charge typically lasts several days depending on use.

What is NFC used for?

NFC lets the ring act as a contactless key or tag where supported. Check that your phone and the service you want accept ring-based NFC.

Will a 20 mm ring fit my finger?

20 mm is the band width, not the inner diameter. It is wide and suits larger fingers; try the sizing kit before ordering, since a loose fit weakens sensor readings.