One of the fundamental and depressing truths that young people stumble upon when reading not only Orwell, but any form of dystopian fiction, is that in all cases, the oppressed vastly outnumber the oppressors. At any given point, though the cost would be dear, the outer party could have risen up and overthrown the inner party.
So, why didn't they?
The Inner Party, Big Brother, Thinkpol, and all other entities in positions of power greatly benefit from people mistrusting one another. We must assume that nothing frightens Big Brother more than people coming together to work with one another. This is evidenced by their enormous security network that constantly monitors them for any aberrant behavior.
The most outright way that the Inner Party utilizes the "divide and conquer" tactic is quite literal. It is heavily implied that the mysterious "brotherhood," the organization said to oppose the party and all that it stands for, does not actually exist. They are merely a prop used to fish out people that might possess anti-party sentiments. This is, of course, the eventual fate of Winston, the protagonist. The party deliberately creates and example of animosity toward itself to separate revolutionary parts of the population and neutralize them. This way, they are never in danger of an all-out rebellion.