On what pretense does O'Brien approach Winston in 1984?
The answer to this can be found at the beginning of Part 2, Chapter 6. At the start of that chapter, O'Brien comes up and starts talking to Winston. The pretense that O'Brien uses is an article that Winston has written in Newspeak.
O'Brien mentions to Winston that he has read this article by Winston in The Times. He says that he felt that the article was written very elegantly. He says that he believes that Winston takes a "scholarly" interest in the language and makes an oblique reference to Symes, who was the expert in Newspeak earlier in the book.
This approach convinces Winston that he was right all along and that O'Brien is sympathetic to his rebellious ideas.