I am somewhat relieved because you seem to be saying that it is not a blowout plug. (Does Mercedes have blowout plugs?)
The coolant is leaking down the side of the oil pan not the front... so it looks like it might be the O ring you refer to.
Also it seems to leak worst at idle after the car has been driven on the highway... but I'm not certain of that.
What has to be done to replace the O ring? Must the intake manifold be removed to make the repair?
We have had snow here and I simply have not had the oportunity to take the air breather off and get a better look.
1993 190E 2.3
2000 Toyota 4x4 Tundra