[Heiko]

It could be anything from a leaky plug tube, vent tube or seal..... If you can't find the source due to the amount of scattered oil, get yourself a can or two of brake cleaner (spray cans) and spray down the motor areas soaked in oil (wait until engine is cool) then drive it for a bit and see where the oil is coming from. Might help to be able to lift the car or have a set of ramps so you can easily access the underside....
The brake cleaner will quickly dissolve the oil and wash it off and then dry the areas.
If the cats are oily then that's probably the source of the smoke, as these get hot enough to burn oil off. You're best bet is to get the car up so you can identify and clearly see the underside and trace the oil...
Good luck with the hunt for the leak...
H