my 'oh' moment was Snape's final memory.
Learning the details of Snape's childhood. Imagining these were the thoughts he put in the pensieve while teaching Harry Occlumency....
Imagining that he left in only those thoughts that showed James in the worst light. Makes you wonder whether he tried to convince himself over the years that he hated James because James was a prat, rather than hating James because James got Lily.
It was a very 'Snape OHHHH' moment for me.
the 'obvious' oh - is that Snape was a good guy. the big 'oh' for me - was the love for Lily. I thought it was just beautiful