We know from the books that the twelve districts are enslaved by the Capitol largely to provide goods for Capitol citizens. The Hunger Games themselves are as a result of the "Dark Days", the rebellion of the districts:
The Treaty of Treason gave us the new laws to guarantee peace and, as our yearly reminder that the Dark Days must never be repeated, it gave us the Hunger Games. The rules of the Hunger Games are simple. In punishment for the uprising, each of the twelve districts must provide one girl and one boy, called tributes, to participate. The twentyfour tributes will be imprisoned in a vast outdoor arena that could hold anything from a burning desert to a frozen wasteland. Over a period of several weeks, the competitors must fight to the death. The last tribute standing wins. Taking the kids from our districts, forcing them to kill one another while we watch—this is the Capitol’s way of reminding us how totally we are at their mercy. How little chance we would stand of surviving another rebellion.
Does this indicate that the districts were oppressed before Dark Days and the Treaty of Treason? Was this what the dark days rebels were rebelling against or did the conditions for the districts get much worse after the first rebellion?