Well, here is how I see it.
I don't think religion has anything to do with opression. I think it has to do with economics and power. Governemts and oppressive groups use religion to justify their grab for resources. It is hard to get an army all lathered up if you tell them that we need to steal the coal mines from a neighboring country. However, if you tell them that the soldiers that they are God's choosen and the enemy is of Satan, well now you have a powerful motivator.
This has been the case throughout human history. Take Joshua, for example. His people didn't have a country, so his army slaughtered every man, woman, and child in hundreds of towns. As far as mass extermination goes, this was more complete than Hitler and Stalin. Of course, this slaughter was justified in the name of God.
Why do the powerful oppress the weak? Because they can.