Oliver recognizes the look of someone who has thrown together a party for a friend and is spending the entirety of it worrying that everything's perfect. He's seen it enough in his mother, though he was lucky enough that the party planning gene didn't really get passed on. (Rather, he let other people plan for him, when he was still in his party phase. Other people, or just Tommy.)

He heads over to Christina once it looks like she's free for a moment and maybe not so stressed out. For her to go to such lengths to throw Tris a party -- Tris, of all people -- meant she had to be a good friend, and Tris' new happiness confirmed as much. The girl seemed brighter than usual with the arrival of her friends, and Oliver knew just how much that changed life in the city.

"This," he says as he approaches Christina, "is a fantastic party."