What is the main cause of my oil leak in my Mercedes c189

Sounds like your front crankshaft seal or possibly oil pan.

Any type of oil can cause problems to areas not designed to have oil on lt. Make concern is to repair your oil leak as soon as possible. Sounds like it could be your camshaft front seal. Or a simple valve cover gasket.

oil in the bell housing can be rear main seal and it can also be input shaft seal in the gear box if manual. The sump bolts on the rear main cap and will leak next to the bell housing indicating a main seal . If auto then the seal on the converter will leak and the oil will leak out of the bell housing. If you are convinced that it is a main seal then check that there is no crankcase pressure issues to make it leak. Change the engine oil and clean the area and if the oils is clean then that will indicate engine or gear box. Put a white chalk line across any oil paths and watch for it to cross the chalk and change the chalk colour.

Thanks for choosing FixYa and welcome. Your computer will not pick this up. It only picks up information from the information sensors in the vehicle. If you think you are leaking oil, I would suggest climbing under the car because you will see it if you do. Over time, bolts can come loose on the engine and need tightened. Oil can leak out of these places. You may want to check the bolts. You may have a bad seal causing a oil leak.

OIL LEAK COULD BE COMING FROM OIL FILTER SEAL. OIL LEAK FROM THE REAR OF ENGINE COULD BE REAR MAIN OIL SEAL.LEAK IN THE FRONT OF THE ENGINE COULD BE FRONT CRANKSHAFT TIMING COVER SEAL LEAKING OIL.

oil pan gasket is on the bottom, the one behind the crank pulley is the front crank seal

Not good, or easy fix. Many things can cause this. Hopefully, its a gasket on the intake manifold. If not that then Head gaskets. From there, you have cracked heads/block. If you have an oil cooler in the radiator, I have seen them leak and cause this also.

hello ,seems your thermostat element should be checked and locate your leaking problem,using a radiator tester could help you easily,also check your oil for contamination due for overheating,ty

No it sounds like you have a transmission leak, oil will be going onto the exhaust causing the smoke and lack of oil will cause the trans will slip.

The dipstick tube can leak oil, and massive oil loss will damage the engine, causing it to overheat in the last few minues of its life.  
Mostly likely that your heating up problem is low coolant/leaking coolant, a thermostat, clogged radiator, collapsed hose, bad cooling fan clutch, or head gasket.