To oppress means to keep (someone) down by unjust force or authority. To repress is (1) to hold back, or (2) to put down by force. Suppress, which is broader and more common than the other two, means (1) to put an end to, (2) to inhibit, and (3) to keep from being revealed.
There is some crossover between these verbs—and suppress often covers all three words’ uses—but oppress usually applies to the mistreatment of a person or group by a more powerful one, repress usually applies to emotions or urges or refers to the violent quelling of political movements, and suppress usually applies to information.
Madison, in particular, worried that a majority might oppress minorities, and that elected representatives might legislate out of “passion”. [The Economist]
They tout arguments about the existence of a monarchy being a root cause of oppression, economic decline, and no doubt soon global warming. [Telegraph]
If women are being coerced or oppressed, they should be supported and empowered by society to make their own choices. [Calgary Herald]
I have come to believe that seasoned reporters seem to somehow repress their natural instincts. [The Age]
Since then, the protesters, the press and human-rights organisations have suffered increasingly violent repression. [Guardian]
Anger, especially in men, is often an undiagnosed sign of depression and repressed grief that needs a healthy expression and healing. [National Review]
Several candidates have complained their ridings are being targetted to annoy voters and possibly suppress the vote on election day. [CBC]
[T]here is no evidence he has falsified or suppressed data. [Washington Post]
In 71BC, the Roman suppression of the Spartacus slave revolt culminated in mass executions. [Daily Mail]