That's because every oppressor is oppressed. We try to segregate people into these two groups, but the fact is that every person is capable of oppression, and every society is guilty of it. The history of human civilization is a story of people who were oppressed that went on to oppress others to continue the cycle.

Our texts don't do a good job of documenting this -- they just designate all losers as oppressed and all victors as oppressors.