What's the fat content in your current feed? and what is your target??
I've seen duck diet recommendations with fat contents up to 5% - in which case, add as much Oats as your want, since they have less than 5% fat, typically. The various "all flock" and "flock raiser" feeds run 2.5% (Nutrena All Flock) to 3.25% (Kalmback 20% Protein Flock Maker) to 3.5% (Purina Flock Raiser). For comparison, both Purina and Nutrena have less fat in their "dedicated duck" feeds. 2.3 and 2.7%, respectively. Even mixed 3:2, you would be under 4% fat with the Purina Flock maker, which I consider acceptable - BUT ***I have made no study of duck dietary needs, apart from Niacin, EVEN THOUGH I raise ducks***
My ducks end up on my table, none live long enough for me to be terribly worried about fatty liver, and I'm not making foie gras. My layers get 18 months, the boys get 3.
My feed attention is more on my chickens.
Just so you are aware that I lack for expertise on this subject.