Berries! I had been adding them at the end ( after cooking) but lately I put them in my oatmeal about one minute before it's done. The berries then get all warm and cobbler like.
Mashed banana is good stirred into oatmeal and so is canned pumpkin with cinnamon!
I was going to say berries, too - I use wild blueberries or mixed berries with cherries. It's my staple. (Sometimes I add nuts instead but that adds a lot of calories.) I have also used protein powder - chocolate is a good one but I have no idea what brand it was now. Again, adds about 100 calories but vastly increases protein content, if you ever need that boost.
I use steel-cut oats; I cook up a batch and then refrigerate or freeze in individual portions for heating later.
I have oatmeal usually at least three days per week. I always have blueberry oatmeal with 1 tbsp peanut butter and a sprinkle of cinnamon. The protein keeps me nice and full until my next meal, and balances out the carb heavy oatmeal.
I use to get hungry at night, so I have oatmeal at bedtime.
I make one envelope of plain instant oatmeal, and a 100% whey protein vanilla shake (made with 8oz nonfat milk). I pour part of the shake on my oatmeal, drink the rest. Total calories 300, protein 35g, carbs 34g, fat 4g fat.
I added honey and raisins today- it was like cookie dough. And I know what that dough tastes like because I ordered a tub of the pre-made stuff from my night last Thanksgiving and never baked a single cookie, yet the dough just disappeared... ;-)