Well, I think Orochimaru's goal, constantly is to gain power. Eventhough we don't know what the rinnegan is for sure, it would be safe to say the sannin knew what it looked like at least. Forget about all the theories I spit out, but keep in mind Orochimaru could very well know about all this ahead of time, and his being inside Sasuke is a fact. The fact that nothing else has been said about this in particular brings suspicion about what's going on inside Sasuke. Is he that powerful?. Orochimaru knew Sasuke hated him and aware that their relatioship was of mutual use. For all of Orochimaru's failures he is still an intelligent warrior, and extremely inmoral, with lots of hidden intentions.