Most likely Dave is correct but this is not a symptom to be taken lightly. I wouldn't drive the car until you have it checked out. I would flatbed it to the dealer and don't let them start it until they have checked the sending unit.
A couple of obvious things - but neither can be done without starting the car. So, maybe you have done this previously.
1. Check the oil level. This should be done within 1 minute of turning the car off.
2. When started cold, do you have oil pressure? When cold, these cars will run higher oil pressures and even a tempermental sending unit would likely show pressure cold.