A racist slur and schoolyard name based on racist stereotypes wrongly assuming that intelligence, articulateness, dapperness, and manners are traits of whites and not blacks. Therefore, a black who possesses these traits is an oreo, white on the inside and black on the outside. A corollary slur is banana, yellow (Asian) on the outside and white on the inside.
That black guy is smart, successful, well-mannered, and speaks standard English. He is an oreo.