Low Weight

I can't really understand exactly what you're saying or what your diet is. However, you'll want to eat calorie-dense foods and foods with healthy fats.

Try dried fruit and nuts as a snack, or guacamole on chips or peanut butter on crackers. Try drinking whole milk instead of low-fat. If you can afford it, drink something like Ensure or Carnation Instant Breakfast between meals.

You need more than oats. You need to add some (healthy) fats and carbs to your diet. Have you looked at any nutritional supplement shakes like Ensure? How about eating nuts or Baby Bel cheese as snacks?

I wouldn't worry about it at 28.
Give it a few more years and by 32 you'll probably be worried about losing a few lbs and it won't be an easy thing to do.

It's hard to know what your weight is, as we don't use the metric system in the U.S. and I don't kow how to convert kilograms to pounds. While oatmeal is healthy, it doesn't contain any real fat. I'd try a nutritional supplement like Ensure, some protein bars, avocado if you like it. Good luck