Depending on the number of calories required, individuals need between 3 and 11 one-ounce servings of food from the grain group everyday, and at least half of those grains should be whole grains.  Since 1/2 cup of cooked oatmeal is a one-ounce serving, you are eating three grain group servings a day in the form of oatmeal, a whole grain.  Your choice is entirely compatible with healthy eating and will not hinder your weight loss as long as you stay within your calorie allowance.