Oppression was the cruel or unjust treatment or control of others. One who caused oppression was known as an oppressor.
In 2373, Garak lamented the name tag he was given at a Bajoran conference on the Occupation of Bajor, which read, "Elim Garak, former Cardassian oppressor". Jadzia Dax questioned whether he would have preferred it to read "former spy" instead. (DS9: "Things Past")
In approximately 3074, a Kyrian arbiter claimed that Tedran died on the USS Voyager roughly 700 years before as a victim of a conspiracy to oppress their people. The weapon used to kill him, and who fired it were beside the point, as it didn't change the fact that her children couldn't attend the same academies as Vaskans, or that they were forced to live outside the city center. (VOY: "Living Witness")