O is for Occlumency
The ability to see into the memories of another person. I doubt we will ever forget the battles that Snape and Harry had in Order of the Phoenix in Harry's attempt to block out Voldemort from his mind.
Severus Snape: "It appears there is a connection between the Dark Lord's mind and your own. Whether he is, as yet, aware of this connection is for the moment unclear. Pray he remains ignorant."
Harry Potter: "You mean if he knows about it then, he'll be able to read my mind."
Severus Snape: "Read it, control it, unhinge it... Used properly, the power of Occlumency will help shield you from access or influence. In these lessons I will attempt to penetrate your mind. You will attempt to resist. Prepare yourself! Legilimens!"
Find out more here.
Previous A-Z posts:A is for Animagus
B is for Boggart
C is for Crumple-Horned Snorkack
D is for Diagon Alley
E is for Epilogue
F is for Forbidden Forest
G is for Galleons
H is for Hogsmeade
I is for Inferius