It is possible to replace the oring with the transmission in place, but it is a tight spot. There are a few pictorials available, dieselgiant comes to mind, but I can't help you search for them just now. One tip I can add that I don't recall seeing in the pictorials is to liberally grease the replacement oring to get it to stay in place when putting the cover back on. Use a thick grease, something about the consistency of Vaseline.
Here is the link. You don't need to go beyond just replacing the red orind as show.
http://www.dieselgiant.com/Mercedesb2pistonsealreplacementl.htm