The meeting that O'Brien has with Winston is made to appear as "natural" as possible, but for Winston, there is absolutely no mistake whatsoever in O'Brien's intentions. He is making contact with Winston for a specific purpose: as part of the conspiracy against Big Brother, O'Brien is the person who is making contact with Winston in order to give him O'Brien's address so that he can go there later on and find out more about how he can get involved in the fight against Big Brother. Note what Winston says:
There was only one meaning that the episode could possible have. It had been contrived as a way of letting Winston know O'Brien's address. This was necessary, because except by direct enquiry it was never possible to discover where anyone lived.
Even though it would perhaps be expected that Winston should feel excitement, as the chapter ends, he only feels an inevitable sinking feeling as he foresees his end at the Ministry of Love. Winston feels that he is "stepping into the dampness of a grave" as he has his conversation with O'Brien, and what makes this feeling worse is that it was an expected grave that he knew was ready and waiting for him to fall into it. Even at this early stage in his attempt to rebel, Winston Smith seems to feel that his defeat and death is inevitable.