Oz has faced heavy criticism after a letter was sent to Columbia asking them to remove him from his position (isn't he tenured? Unless he intentionally runs over a minority handicapped woman in the parking lot, good luck getting rid of him) followed by a USA Today editorial from fellow Columbia academics wishing he would dial down the miracle vegetable talk.
Oz went on the offensive in his program, dismissing his critics as pro-GMO, which sells with the anti-vaccine, anti-GMO, pro-Whole Foods contingent but almost no one else, yet he was more contrite in his NBC mea culpa, assuring the world that his stethoscope, surgical scrubs and the use of "Dr." in the title of his show does not make it a medical show.
This only led to John Oliver aiming the humor guns at him again, calling him the "worst person in scrubs that has ever been on television, and I am including Katherine Heigl in that..."
It's unlikely Oprah made the decision based on any of those issues, since clearly the Oprah audience and the Oz audience are the same. It must be that it didn't make any money.