I’m not quite sure what you mean by ‘Oxford’ but the phrase is certainly in OED and, as you can see from the earliest cite, it’s been with us some considerable time.
dumb down 1. trans. To simplify or reduce the intellectual content of (esp. published or broadcast material) in order to make it appealing or intelligible to a larger number of people. Also in extended use.
1933 H. T. WEBSTER in Forum Dec. 372, I can cheer, too, for the Hollywood gag men in conference on a comedy which has been revealed as too subtle when they determine they must dumb it down.