O'Brien knows about every "criminal" activity that Winston has engaged in to this point — even something as "minor" as Winston's memory of the photograph of Aaronson, Jones, and Rutherford upon which he had been basing much of his evidence that the Party was deliberately changing history. Even without the physical photograph, the image still exists in Winston's memory, and O'Brien uses this image as an example of Winston's inability to want to change for the better — the better of the Party.
O'Brien tells Winston that Julia has betrayed him, but there is no evidence in this chapter to prove that it is true. Winston has not betrayed Julia, and that fact is the only thing that keeps him from being "reintegrated" — the only thing keeping him human.
