Who is brought Winston into the cell with Winston and why does he think he's there?

Winston is brought into the cell with O'Brien, a high-ranking member of the Party whom Winston initially believes to be a covert ally against the oppressive regime. He thinks O'Brien has been secretly opposing the Party and that they share a common goal of rebellion. However, Winston soon realizes that O'Brien is actually there to interrogate him and enforce the Party’s ideology, revealing the futility of his hopes for rebellion.