The idea of oppression and its effects on those who oppress and those who are oppressed is carried throughout the novel. Many examples are given in the other answer, but we should also look at the power that those oppressors seemingly function from. We recognize the treatment of the poorer characters as well as the black and "different" characters as various forms of oppression. The oppressors want control over those and use racism, and segregation as their tools. The community stayed out of the Ewell business until it was brought forward and testified about at a trial that was based on lies. Tom Robinson felt sorry for Mayella; he is ultimately convicted because he tried to rise above the oppressiveness evident throughout the novel. He is looked at as incapable of such sympathy because of his skin color. Boo Radley is kept hidden and indoors, out of the community's sight. Ironically, the little gifts and "reaching out" to the children, becomes his only path above the oppression he must deal with. His break out of the oppression, when he saves Jem and Scout's life, is one more example of those who are oppressed rising above their set station despite the odds against them.
Yes, oppression most certainly is evident in this novel. First of all, we meet Scout, who, ironically, is oppressed by her teacher and singled out because she can read too well! Next Boo Radley is oppressed because he is different; he is a recluse and the town has made up gossip about his so-calledcrimes that have made him strange. Ms. Dubose is oppressed by the drugs she has taken to relieve her pain, and at the end, dies a hero's death without painkillers. Obviously, Tom Robinson is oppressed because of his skin color and the racism in the town. Mayella is oppressed by her father and her economic status. On a deeper level, Jem is oppressed by the process of growing up, his own rite of passage to become a man. Atticus is one character who will not be oppressed because he is driven by his integrity and conscience.