One of these days during a review, I should try and see if Duolingo accepts "oes" instead of "ydw." It's a common way of answering "eisiau" questions in parts of North Wales.
We accept Oes/Nac oes when used in conjunction with, here, llefrith and isio - the three variants tend to go together. They are taught in that combination in the 'north' variants of the introductory DysguCymraeg courses.