During a two-minute hate session, Winston is revulsed, as he always is, when everyone starts chanting "B-B!" for Big Brother. This mindless, hysterical chanting, with the pause between the first and second B, always makes Winston go cold inside. Yet he always goes along with the chanting, as anything else would be too risky. This time, however, he pauses for a few seconds, and at this moment O'Brien, who has been polishing his glasses, looks at him. Their eyes meet for "a fraction of a second" and Winston is sure:
that O’Brien was thinking the same thing as himself. An unmistakable message had passed. It was as though their two minds had opened and the thoughts were flowing from one into the other through their eyes. ‘I am with you,’ O’Brien seemed to be saying to him.
Winston's thought tumble on excitedly in this vein. He is sure that he and O'Brien have had a meeting of the minds and that O'Brien is "with" him. A few moments later, Winston wonders if he simply imagined the whole thing. Then, on the basis that there might have a been a split second of connection, Winston becomes excited by the idea that a Brotherhood working to overthrow the regime might be real.
We can see from this brief interaction how lonely Winston is and how hungry for a kindred spirit who shares his views. He is willing to grasp onto anything that might offer hope of an alternative way of life.
As we will later find out, Winston simply projected his own desires onto a man he wished could be his father figure.