Is it the oil pressure sender or the guage (or nothing)?
 

On my 1985 500SEL, the oil pressure guage jumps to the top as soon as the key is in the number 2 position and does not move until the key is turned off, whether the engine is running or not. What voltage range should there be to the guage? Could it be the sender?

Either the connector has come loose or the sender is toast.

I concur with Jeff. You should have continuity between the sender post and ground with the engine off. Either the connection is really, really dirty or the sender is shot. Just went through this on the 560, turned out the sender was bad.

Thanks alot I will look at the sender and let you know