Yes, there are several studies. One is this [1]. I have not tried the Oura 4 myself but once I test it I can compare with other devices. Clearly sample=1 but I found big errors with some watches.

[1] "Accuracy Assessment of Oura Ring Nocturnal Heart Rate and Heart Rate Variability in Comparison With Electrocardiography in Time and Frequency Domains: Comprehensive Analysis": https://pmc.ncbi.nlm.nih.gov/articles/PMC8808342/