I think Oberyn purposefully didn't kill him because he wanted the confession. Therefore the wounds he made were no doubt planned in non leathal parts of him. His injuries would have been bad and I wouldn't necessarily expect him to last long but who knows.
Other characters have survived some pretty nasty blows (Jon comes to mind) and returned to full fitness.
Yea I'm still not sure whether he'll be alive or not, Oberyn said that he was dying after stabbing and said he couldn't die until the confession but then again it is the Mountain so normal rules may well not apply to him
