In order for oppression to occur in a society, the first thing that must exist is a population that does not really care about the oppression, at least when the oppression first starts. Once the oppression is in place, it can stay even if the populace starts to care more.
For a population not to care, I think a few things (at least some of these if not all) need to be in place:
- Conditions in the society have to be bad so the people will want change even if it comes with oppression.
- The people must be poorly educated.
- The oppression must start with people who are not popular and only later spread to include everyone.
Oppression by definition is the deliberate discrimination, legal or otherwise, against a minority group. For this to take place in a society, there must be an identifiable minority group. Keep in mind, this could also be a minority power group as well, such as a third political party, racial or ethnic minority, religious group, etc. In other words, your society must be somewhat diverse, with a power group enjoying privileges.
Secondly, the law or the social mores of the time and place must be accepting of the oppressive actions, it must be tolerant of the discrimination taking place so much so that the courts and the government do little or nothing to stop it.
Lastly, the lack of a legal framework of democracy, where the target group is either disenfranchised, or has few legal rights in the first place means that oppression is much more likely.