Ducks, like chickens, need a maintenance feed of at least 15% protein. You can feed chicken feed, allflock or chick starter. A laying female will do fine on layer feed. I prefer to feed mine pellets, as they tend to try and choke themselves on mash, and have nearly as much trouble with crumbles. You'll probably have to supplement B-vitamins, as most chicken feed doesn't have enough (especially niacin) for them. Just buy vitamin B-complex (powder, liquid, or pills) and add it to his feed occasionally. Some people use brewer's yeast, but that's rather expensive, comparatively.
Chop ain't gonna work. Ground oats + ground corn is rarely more than 11% protein, lacks many of the vitamins and minerals a duck needs, and is mostly intended for huge ruminants that can actually get nutrition out of fibrous husks. Ducks are omnivores, not herbivores. My free-range ducks eat snails, mosquito and dragonfly larvae, newts, tadpoles, small frogs, and any other small animals they can catch. Corn and oats simply do not have all the nutrition that they need, though you could keep them alive on it for a while.