What effect does Big Brother have on Winston in 1984, and how is his obsession with O'Brien different from his attitude toward Big Brother?
In 1984, Winston has a very negative attitude toward Big Brother. In fact, he hates Big Brother, and this is shown clearly in part one, chapter one when he writes "DOWN WITH BIG BROTHER" over and over in his diary. This attitude only changes at the very end of the novel after his incarceration in the Ministry of Love, especially in Room 101, where Winston is tortured so much he finally comes to love Big Brother.
In contrast is Winston's feeling towards O'Brien. He is obsessed with him because he feels that O'Brien might be a potential ally who shares the same hatred of Big Brother. This feeling is based on a dream that Winston has in which he hears O'Brien's voice. Winston's obsession with O'Brien reaches its climax in part two when he is initiated into the Brotherhood. It is only later, however, that Winston learns the truth about O'Brien: he is not an ally or member of the Brotherhood, but, in fact, an agent of the state.
Winston holds a sort of contempt for Big Brother. When he sees posters of Big Brother, instead of being moved with respect or fear for the figure, Winston questions the existence of him. In the end, of course, he succumbs to the torture and brainwashing and proclaims his "love" for Big Brother.
In contrast, Winston adores O'Brien. Even in the middle of being tortured by O'Brien, Winston thinks he loves him. While O'Brien certainly represents the Big Brother mindset, he is a living, breathing entity with whom Winston has an intellectual connection. That connection is what allows Winston to be deceived and eventually completely brainwashed.