How do themes of marginalization and oppression show in the novel Invisible Man

Fortunately, there are many examples from which you can choose in Ralph Ellison's massive novel. Here, I'll only provide you with a couple to give you a sense of how to think about how these themes work in this novel.
Marginalization is the practice of excluding people from certain rights, opportunities, and privileges. In the novel, the nameless narrator is not only marginalized in American society due to white supremacy, he is also alienated from the institutions that he trusted to protect him--a historically black college, his Harlem community, and the Communist Party.
I think that the episode with Dr. Bledsoe, in which the president of the narrator's college (modeled on Ellison's alma mater, the Tuskegee Institute) berates him for making the mistake of taking Mr. Norton, an important donor, to the old slave quarters, where he meets Trueblood. The narrator believed that he could improve himself at the college and that Bledsoe could serve as a mentor. He is disillusioned, however, to realize that the college president has no interest in helping the narrator or any other black student. He uses his position to acquire privileges and prestige for himself and has no qualms about stepping on others, particularly other black people, to get what he wants. The Bledsoe episode is an interesting example because it reveals how marginalized people are sometimes willing to marginalize others to gain some privilege in an oppressive system.
In regard to oppression, I think that the Battle Royal chapter, which is frequently extracted as a short story, provides a very explicit example of how black men have been economically, sexually, and politically oppressed in American society. It also reveals the complexity of power dynamics; a white woman is rendered a tool of white male supremacy by remaining passive in the face of racism but, she is also complicit with her objectification.