Combine beef, oatmeal (or breadcrumbs), milk, egg, onion, salt and pepper.
Make sure it is mixed well and place in pan.
Mix ketchup, brown sugar and mustard in a small bowl and pour over the top of the meatloaf. Save a little to add to the top at the end of baking.
Bake at 350 degrees for 1 hour and 20 minutes.