After being arrested by Thought Police, O'Brien visits Winston to torture Winston psychologically giving him physical pain.
Explanation
The characters Winston and O'Brien come from the story '1984' by George Orwell, where a totalitarian system rules the Oceanic which is the previous name of Great Britain.
Winston is the protagonist of the story.
He along with other residents were forced to embrace the ruling of the Party over their basic rights.
Winston considered O'Brien as a hero to disrupt this system but the truth was O'Brien was a firm believer in the policies of the party and the Big Brother.