God, I love Supernatural finales! They are always just incredible, and this one is easily top 5, at least for me.
The drama, the intensity... the parallels between Sam in season 4, vs Dean now (being strung out on the power, almost overwhelmed by darkness, on the verge of being demonic, the innocent who was sacrificed (ie: the nurse for Sam, Rudy for Dean), Dean alone and scared, Sam desperate to save him no matter what), not to mention Jared and Jensen's phenomenal acting. That scene in the restaurant is freaking haunting, and is what should have happened in season 4.. Sam not pushing Dean away, but embracing him, loving him, accepting him no matter what and doing what is best for Dean... that scene is the healing that I think Dean has needed since season 4, it just took this long to get. Also, a subtextual foreshadowing for the Darkness, and what she ultimately ends up needing the most as well.
In my opinion, there is no doubt that Sam won this episode. Rowena may have cast the spell to free the darkness, but it was because of Sam's efforts, and relentlessness to save his brother. Sam's love and devotion to Dean also snapped him out of just how far he'd fallen, and brought him back. Snapped him out of Death's logic. Even though Dean did kill Death, it was Sam who ultimately checkmated Death, by agreeing to the plan, and accepting his 'fate' instead of resisting it (and don't even get me started on my headcanon that this is actually the exact outcome Death wanted, because he knows what Chuck is, and what will happen between Chuck and the Winchesters, and that this is the start of the Winchester's on that particular path. And, as we have seen, Death has always admired and respected the Winchesters, and helped them however he could before)
The thing with Oscar, they actually set up a few episodes back. This isn't the first time Crowley meets with him, and he found out about him from Olivette, after Rowena turned her into a hamster (there was a scene where he was speaking to her, trying to find out if Rowena had a weakness, and then another scene of Crowley and Oscar meeting for the first time).
Also, what triggers Dean to kill Death, is, while Sam is on his knees, after he shows Dean the photos, Death says "Do it... Or I will." Once Death issues that threat against Sam, you can actually see the change in Dean's eyes. Killing Sam himself (at this point) is one thing... but not even Death himself is allowed to threaten Sammy