Gothic was also a single player game but a group is apparently working on making a coop/multiplayer version. I'm wondering how they did it and if the same can be done for OB (or MW or FO3, etc for that matter). http://www.gothic-x-project.de/

Someone already has.

http://csusap.csu.edu.au/~mloxto01/

pity it didnt seem to have taken off or gotten much attention. the gothic one appears to be ongoing.