They were not oppressed. That is democracy. The people decided that it was in the best interest of america if black people were forced to slave labor. They voted on this and the democratic process prevailed.
This is what you believe in is it not? If i want something, but the people vote against me, then i have to submit to the people. Well, unfortunately for your ancestors, the people voted that slavery was a good thing. Had people of not wanted slavery, they should have contacted their elected representatives and asked them to change it, or vote for representatives that didnt support slavery. That is democracy..... this is how democracy works.
By the way, just because i'm curious.... which political party supported slavery?
I personally believe that all men have rights that cant be voted away from them..... but that's me....