A black person who is said to "act white" because of the way they dress, talk, or act. Someone who does not play in to "acting black" and believes that there is no way to "act black". Someone who typically hangs out with whites, or gets along well with whites.
Sometimes blacks take being called an oreo as an offense, but others might take it as being called a "classy black", someone who did not grow up in the projects so has no particular reason to be someone they aren't in order to fit in well with the black community.
Caleb is such an oreo. He's always going up north to snowboard and lives up in rich country with the white folks.