As others have noted, societies like Oceania arise from humans needing security, whether it be from outside forces, civil conflict, etc. Orwell's proposition is that when people are willing to sacrifice their freedom for security and when they place too much trust in a person or small group of people, they are likely to end up living under a totalitarian regime.
As much as I keep telling myself that this is only possible in the world of fiction, I have a harder and harder time convincing myself of that. As others have mentioned Hitler in Germany is an example of how this can happen. In the world today and the way politicians can spin stories I am not sure it can't happen.
This society is not much different than WWII Germany. People were told that Jews were the cause of all their problems, and the propaganda mostly worked. Rights were taken away little by little, and no one stood up for the right thing...by the time people started getting upset, it was too late to revoke the changes and a healthy dose of fear was already choking the life out of the public. It doesn't seem all that out in left field that a society could end up this way. All it takes is a public which does not dare to speak up and take risks to support what it knows to be right. That is truly scary.
To me, this society arose completely from apathy. When the people were distracted, laws changed. History was re-written. Then, as time kept moving and as outspoken dissenters were "erased" the Party gained more and more control of the people.
Orwell certainly offers that this is probable, or he wouldn't have written this cautionary tale to make us think. As long as societies keep thinking, I think the threat of the people in revolution will keep extreme leaders in check in at least Western societies. But, when we let television control our thoughts, or the words of a leader are taken as truth without fact-checking, then we have come to a dangerous place that looks a lot like 1984.
Benjamin D'Isreali once discussed his Cycle of the Body Politic, which illustrates the continuous cycle of freedom to bondage. I share it with my classes when we discuss people who face repression or some form of a "Big Brother" authoritarian government. Imagine this as a circle - this could begin at any point in the cycle as well, but I'll choose "freedom" as a good starting point:
When I ask my students to identify where the U.S. is right now, they overwhelmingly state somewhere between "complacency" and "apathy".
I think the scoiety in 1984 seems to follow this cycle quite accurately-we see the transition from "dependency" to "freedom" in the novel, it seems. According to Disreali, this is the natural course of nations and cultures. See http://www.fixfreedom.com/cycle/35-ideas/46-body-politic.html for a further description. By the way, I hope I'm not posting any political bias in this response-that's not my intention.
I agree with #3. The majority of dystopian worlds that we are presented to in fiction have come about because of a crisis, war or nuclear holocaust that has radically changed civilisation as we know it into something new and strange. We are never given the direct answer to your question but we can infer that political realities have formed three power blocks in the world. In one, a radical state of communist dictatorship has been imposed that has all but ended the world we call home and replaced it with something dull and grey in its place.
I think that this society came about because of crisis. I think that crises are how society can get moved in radical directions.
When there is a crisis in society (a war, a depression, things like that) you get people who are willing to go along with radical solutions to their problems. You can say that this is how Nazi Germany came to be -- people taking a radical solution to the crisis that arose after WWI. You could also say that a crisis (the Great Depression) totally changed our society to one where the government plays a much bigger role in every aspect of our lives.
So this is what I think happened in the society of 1984 -- there were a series of crises and people wanted more certainty. Because of that, they went along with what the Party was doing.
One of the main tools of Big Brother in molding the society we find in 1984 is fear. Of course people will always desire freedom to make their own choices to see or spend time with the people they choose to be with, to want to voice their opinions, etc. So Big Brother uses various types of fear to maintain control, fear of the enemy (whomever the enemy might be that day), fear of Big Brother itself, fear of neighbors and being turned in, etc.
Because of the desire for some kind of safety and routine and level of comfort, this fear works very well at keeping people in line and allows a powerful state to take control over most aspects of people's lives. Look at North Korea or the Soviet Union for good real-world examples of state-run societies.