I used to get this message frequently in my W208 CLK. Apparently there to warn of 'over-filling' oil levels, the sensor is notoriously inaccurate (according to my trusted local indy specialist and 'goes off' for little or no reason. I accused him [jokingly] of using cheap rubbish oil after a service and was told it was a 'known fault' on several MB models.
Check oil using the old-fashioned dipstick to make sure you're OK.....!
The other possibility on an older diesel car is that that the oil is being diluted by excess fuel finding its way into the sump. This not a good thing for the engine. I would buy your own dipstick tool on Ebay so you can keep a permanent watch on the oil level