The Brief Wondrous Life of Oscar Wao begins with poet Derek Walcott's lines: “I have Dutch, nigger, and English in me, and either I’m nobody, or I’m a nation.” So, Diaz's focus is on cultural assimilation: we are more alike ("a nation") than we are different ("nobody").
In the novel, Diaz focuses primarily on black Dominicans, black by skin color, descended from Africa, but living in a culture that choose lightness over darkness and straight hair over kinky. Males seem more at home in their skin than females. Beli abhors her blackness. She is a victim of the sexist male culture (machismo) and the racist culture (light-skinned against black-skinned Latinos).
The novel focuses on the New World immigrant (Hispanic / Latino / Afro-Caribbean), the largest wave of contemporary immigration. Diaz plays with the stereotypes of Dominican male virility, using Yunior as his meathead narrator. Oscar asks his roommate one night:
"I have heard from a reliable source that no Dominican male has ever died a virgin. You who have experience in these matters--do you think this is true?"
The book also focuses on the "ghetto nerd" culture and all its sub-cultures. Yunior knows the dorkiest of sci-fi and Marvel comic book references:
"My shout-out to Jack Kirby aside, it's hard as a Third-Worlder not to feel an affinity with Uatu the Watcher."
Here are other cultures assimilated in this pastiche:
•Science fiction: e.g., Isaac Asimov
•Fantasy: e.g., Lord of the Rings
•Comic book: e.g., Spider Man
•Literary: e.g., Oscar Wilde
•Pop Culture (Media): e.g., Dr. No, Land of the Lost
•Dominican History: e.g., Trajillo
No one could possible read Oscar 50 years from now and discern all the esoteric cultural allusions without footnotes or marginalia. In this way, Diaz makes us all feel like an immigrant or a minority, in some way or another.
Also worth noting is the "Annotated Oscar" website, which explains all the sci-fi, comic book, and pop culture allusions: