Combine ground beef, oatmeal, milk, egg, onion, salt and pepper in a medium sized mixing bowl. Using your hand, incorporate everything into the meat. This only takes 30 seconds or so, you do not want to over mix.
Transfer meat into a large loaf pan and press it down evenly into the pan.
Mix ketchup, brown sugar, and mustard in a small bowl. Pour over meatloaf.
Bake at 350 degrees for 1 hour and 20 minutes.
Remove from oven and drain off excess grease. Let stand for 5 minutes before serving.