White, Christian, heterosexual males whom, arguably, caused most of racial, gender, and sexually-oriented injustices seen today.
Though always debatable,it is now pretty common knowledge that, most of the time, whites are known to be the most racist toward other ethnic groups.
Christians, while not a bad thing in itself per se, are often accredited with being blind zealots. They tend to persecute fellow mankind more than love them...
Heterosexuals tend to be commonly viewed as being prejudiced against gays... either by hate or ignorance.
Men have commonly been portrayed as intolerant to respect womens' rights and value them as an equal-- especially on vocational/financial terms.
Combine all these attributes, and you have The Oppressor.