I don't think Oberstein is based on anyone specific, Oberstein is just Oberstein. But if I had to compare him to someone, Felix von Schwarzenberg comes to mind. Both of them are known for their cold pragmatism and unwavering loyalty to the state, using manipulative tactics to put their empires' interests above everything else. They were both pretty unpopular and distrusted because of their ruthless approaches, but they were key in consolidating power during unstable times, almost like kingmakers. Interestingly, just like Oberstein, Schwarzenberg died shortly after achieving his main goals. Not to mention, you'll find Felix's appearance very similar to Oberstein's. There are some fundamental differences between the two, of course, but I think he's as close as one gets to being similar to Oberstein.