Julia is the youngest of the three and doesn't remember a time when Oceania was not in the grip of the Party, Big Brother and the propaganda apparatus that surrounds her. She has never been especially important and doesn't have a context for envisioning a life outside of a totalitarian state. Her viewpoint is pragmatic: she wants to work the system to her advantage so that she can have some fun and pleasure in life. She doesn't care, for example, whether or not the party invented the airplane: her focus is on the here and now.
Winston, on the other hand, remembers the time before the Party took control and know how it has tried to alter reality. His opposition to totalitarianism is primarily idealistic rather than pragmatic. It bothers him that his job requires him to alter the historical record on a daily basis. He realizes that life has become narrower and much more controlled for the average person and this bothers him. He would like people to have freedom to think for themselves, know the truth and develop their human potential.
O'Brien, as he will reveal to Winston in Minilove, is motivated purely by the desire for the power and believes he and others in his elite group can achieve almost ultimate power--godlike power to determine truth and reality--through the Party. While he calls Winston insane, it is clear that he is the insane one, driven by an overwhelming lust for absolute control. Triumph is the emotion he wants to experience: he wants to crush other people and make them suffer so he can be sure he has power over them. As he says, if a person isn't doing something that makes them suffer, you can't know if they are doing it from their own will or because they have no power to resist. O'Brien is the face of totalitarianism, driven by a lust for power or as he puts it, the boot stamping the human face.
Both Julia's pragmatic desire for ordinary human pleasures and Winston's more idealistic desire for truth and freedom are crushed by O'Brien's will to power.