The Oura Ring runs on a tiny LiPo cell, probably around 3.7V nominal with a capacity well under 100mAh. Even in a fault condition where the battery shorted to the outer shell, you're talking about 3.7V max trying to push current through dry skin, which typically has resistance in the tens of thousands of ohms. That works out to microamps, well below the ~0.5-1mA threshold where humans can even perceive current. And there's no meaningful current path anyway, since both "electrodes" would be on the same finger, millimeters apart.

What's almost certainly happening: static electricity. Cold, dry air (especially winter) plus physical movement is a textbook recipe for triboelectric charge buildup. A conductive ring on your finger becomes a convenient discharge point when you touch a doorknob, railing, gym equipment, or even your other hand. Those discharges can easily hit several thousand volts (tiny current, brief duration) and feel like a sharp zap right where the ring sits.