All I can find is a hint from this story:
One user of ours in Finland was traveling in early March. His scores were normally in the 80s or 90s and he noticed his readiness score dropped to 50 and that caused him to get tested.
My experience suggests that a 20+ point drop indicates either a significant cold, unusual stress, or obvious measurement error. I had a clear-cut cold in November that caused my readiness to drop by 50 points over a period of 2 days. I've had no other times, in the year that I've been wearing my Oura, when I suspected I had a cold or had an unexplained 20+ point drop.
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.
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?