Oes gen ti gar? Oes. "Do you have a car? Yes I do."
Oes llyfr gyda ti? Oes. "Do you have a book? Yes I do."
Oes ci yn y tŷ? Oes. "Is there a dog in the house? Yes there is."
If you ask with oes, the "yes" answer will be oes.
The answer to Allwch chi ganu? would be Gallaf. -- again repeating the verb. Oes? Oes. Allwch? Gallaf. Dych? Ydw. (Is there? There is. Can you? I can. Are you? I am.)