I think it would be fine to give them oatmeal or other treats.
A few years ago, when I was raising CX, I accidentally got a 50 bag of crimped oats, instead of whole barley. Since I had already cut open the bag, I didn't want to take it back to the feed store. I just started mixing the oats into their fermented feed -- maybe 10 to 15% by volume. They all did fine. I've also been pretty free with with given them old produce, grass clippings, sprouted wheat and barley, and weeds. In my experience a CX will convert just about any type of food into weight gain. The only things I would avoid is giving them were very salty foods (which are not good for chickens) or very strong smelling types of food -- fish, garlic, etc. -- as I would worry it might end up impacting the meat flavor.