Can eating oatmeal cause belly fat?

No it does not, it is very healthy for you and cuts down on cholesterol.

Accumulation of body fat is caused by the number of calories consumed exceeding the number of calories used (burned) by the body. Where your body accumulates fat is determined by your genetics.

You should know how to monitor your calorie intake by reading the labels on processed food and for fruits, vegetables, meats, and seafood, which use the USDA National Nutrient Database for Standard Reference.