Question: If you criticize Obama, you're a racist
The statement in and of itself is more opinion than a specific error in reasoning, although there are studies that examine the racist tendencies and roots of Obama criticism (Psychology Today). Certainly, it cannot be claimed that all Obama critics are inherently or ideological racist. However there are numerous slippery slope arguments than can develop from this kind of exaggerated generalization: Fallacies of Composition or Division come to mind, depending on which way you frame the claim, Whole to Part, Part to Whole, as well as ad hominem (guilt by association) or even a potential Transfer Fallacy rears its ugly head when attributing the quote to Oprah without verification.
I would argue against Questionable or False Cause as the fallacious culprits here because there are formidable studies that bolster the claim - so it is not entirely unreasonable. Nevertheless. there is no real cause or logical premise here - only opinion.
Logically, it is really not important who said it, but whether it is an error or blatant deception in reasoning.