Why do people call Oprah white?

i sincerely do not get this! 50 cents called ehr oreo and others call her white? some say she acts white? what the hell is acting white? what's acting black? i dont get this. i think she helping a lot of people with her donations especially africa but people are still dissing on her....as mulatta [half white and half black] i have no freaking clue why they would call her white or saying she is acting white?? i dont!