Chickens are actually very good at regulating their own nutrition and food intake when given healthy food choices. It's instinct. It might look like they are eating lot or eating quickly but their digestive system is actually designed for this, as their ancestors lived on a scavenger diet.
The food will collect in their crop -- an organ meant to be able to expand to hold a large meal, like a storage tank-- and then will slowly make its way through their gizzard where it is ground up by little rocks (grit!) and then digested through their intestines.
Most birds will eat a lot in the morning when they are the hungriest and then graze all day. They go to bed with a full crop and digest food through the night, waking up with an empty crop and feeling hungry. rinse and repeat.
I would not worry about them over eating. And, as long as they have a balanced diet they should keep themselves at a healthy weight even when feed freely. Also, I am not sure how many birds you have but 2-3 cups of oatmeal as an occasional snack does not sound like a huge amount? I just wouldnt give them the instant kind that is full of sweeteners. Any kind of plain oats would be fine.