The Rockefeller Neuroscience Institute seems to have developed an app that can detect COVID-19 3 days before symptoms based on Oura data but the app isn't publically available. How should a user that doesn't have access to the app interpret his Oura measurements to know when they should self-isolate or get tested?
The Finnish user in early march is likely Petri Hollmén. I wrote a bit about him back in Using the Quantified Self paradigma for COVID-19 . Petri mainly got convinced about the decision to get tested mainly based on a nightly temperature rise from Wednesday to Thursday when he was infected on the weekend.
Unfortunately, Petri wasn't willing to share resting heart rate and heart rate variance (HRV) so I don't know. I and others asked him on his Facebook post where he described the episode but he didn't reply.