Olly will kill John
I highly doubt anything is going to happen to Jon. It just wouldn't make sense for him to die.
For one, we've already had the Red Wedding. We don't need a second Stark getting betrayed by his own men and killed, it would just feel pointless. Besides, Jon is basically the only main character in the Wall storyline. Not that being a main character in this series really means anything, but he's the ONLY main character really left at the wall. If they kill him off, that storyline still has to continue, unlike it did with Robb. The WW are still coming, who are we going to focus on then? Sam? Thorne?
Plus we already had the scene between Sam and Olly which seemed to me like it was setting up him NOT betraying Jon. Even Thorne, in the last episode, opened the gates for Jon, and his remark to him didn't really sound like a threat. ("you've got a big heart, but it's going to get us all killed") Sounded more like Thorne understands what Jon is trying to do, but just thinks it's too risky.
Besides, what about what happened at Hardhome? Jon, several of the nights watch, and 5,000 wildlings just came face to face with the WW and an undead army. I can't believe that after bringing that story back to the wall, the nights watch are really thinking about much else. Surely they're not stupid enough to kill their Lord Commander when their fellow watchmen just saw the undead up close and personal. Everything Jons been trying to say is now suddenly proved to be true.
EDIT: You know, if Sansa kills Ramsey, do you think that Reek is going to revert back to Theon? Or is he going to start worshipping Sansa? I have a feeling that Sansa and Reek might be a combo for a while...