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.