Although you do not specify which dream of Winston's you are talking about, I think you are most likely talking about the dream that Winston mentions in Chapter 2. He thinks about this dream after he has been helping his neighbor with her plumbing.
In the dream, he is walking through a dark room. There is someone in the room. As he passes by the person, the person says to him "We shall meet in the place where there is no darkness."
Winston does not know in the dream who that is, but he comes to believe that it was O'Brien who spoke to him.