In 1984, what finally convinces Winston that O'Brien is a member of the Brotherhood?

is finally
convinced thatis a member of the
Brotherhood in Part Two, Chapter Eight.  In this chapter,
Winston andare
invited to O'Brien's apartment. Initially, Winston feels nervous that his

invitation to the apartment might be a ruse, but once he arrives, O'Brien
turns
off the telescreen (as a means of providing some
privacy), and this convinces
Winston that he is indeed genuine.

Moreover, Winston is further convinced in
the conversation that
follows. O'Brien tells Winston and Julia that the servant is

"one of us." This phrase implies that they are all of the same

philosophy, that they are united by their desire to overcome Big Brother.

As
their conversation continues, Winston
is...