How does Big Brother watching over the people of Oceania prevent the people from rebelling?
The very fact that someone is watching over you does something to your psychology. In the least it makes you feel deeply uncomfortable. Even if you are not sure that they are watching you every second, the randomness of the watching is also a great deterrent from rebelling. Now if we couple this with the power of the state, the act of rebellion is even more unthinkable. Finally, we need to consider the fact that the people of Oceania are without power. Therefore, what we have is power and omniscience on the one hand, and powerlessness and nakedness on the other. With these odds, rebellion is almost unthinkable.
Just think about how it would affect you if there were cameras all over your school and your town and your house. Would you ever do anything that was illegal or against the rules? Not me.
So the fact that Big Brother is watching prevents the people of Oceania from rebelling because it means that they'll get killed if they do not obey. If they disobey, they will just about surely be seen. If they are seen, the Thought Police will get them. As we see from what happens with Winston, you do not want that to happen.