Can someone please tell me EXACTLY how the Check Oil light works in a '90 Accord? I know that it is a warning light to tell you when the oil is low...but how does it work exactly? Where is the sensor?

My g/f's '90 Accord is intermittenly showing this light. I have checked and rechecked her oil, and it's fine. Maybe even a bit OVER the full mark (is that the problem maybe?)

Any ideas?