Oatmeal itself has little or no sugar, high fiber, you can indulge and eat as much as you want. It will fill your stomach and make you feel full long before it bombs you with too many calories or sugar.
For example: Bob's Red Mill Extra Thick Rolled Whole Grain Oats is what I eat. The bag says:
serving size: 1/2 cup
calories: 190
total sugars: 1 gram
fiber: 4 grams
protein: 6 grams
total fat: 3.5 grams
That 1/2 cup of oatmeal is cooked with 1 cup of water for a total serving volume of 1 1/2 cups. Go ahead and eat 2 or 3 servings, if you can stuff that much down. Especially if you're going out for a long hike or bike ride. It's all good for you.
What makes "oatmeal" unhealthy is some of the stuff people put in it: syrup, cream, butter, etc. For me, the additives are almond milk, unsweetened cinnamon and berries.