In "1984," what finally convinces Winston that Obrien is a member of the brotherhood?

In George Orwell's novel, "1984" Winston, the protagonist, has long suspected O'Brien of being a member of the brotherhood.  He is afraid to approach O'Brien, but there are certain non-verbal clues that Winston feels he has gotten from O'Brien. Specifically, eye contact in the halls at work.  One day O'Brien stops Winston in the hall at work and speaks to him about an article Winston has written and the new 10th edition dictionary.  O'Brien gives Winston a piece of paper right in front of the telescreen on which he has written his home address. He tells Winston to to stop by sometime and he will show Winston an advanced copy of the Dictionary.  Winston, at this point, is convinced that O'Brien is a member of the brotherhood and this is his way to meet with Winston outside of work without arousing suspicion.  Winston memorizes the address and throws the paper in the "memory hole.

"Winston knows it is only a matter of time before he visits O’Brien. Frightened, he feels a chilling sensation passing through his body as he has the sensation of stepping into a grave."

When he does go to O'Brien's home, he meets there with Julia.  O'Brien turns off the telescreen and now Winston is positive of O'Brien's involvement and thrilled to think that he will finally be able to act against Big Brother.