In a large bowl, combine ground beef, egg, oatmeal, milk, onion soup mix, Worcestershire sauce, yellow mustard, salt, and pepper.

Use your hands to thoroughly mix all ingredients.

Place the meat mixture on a baking sheet.

Shape the mixture into a loaf. Using a loaf pan as a mold can help achieve a uniform shape.

Brush ketchup evenly over all sides of the meatloaf, ensuring the entire surface is coated.

Place the meatloaf in a preheated 375-degree oven.

Bake for 45 minutes, or until the center is fully cooked.

Remove from the oven and let the meatloaf rest for 5 minutes before slicing and serving.