Preheat oven to 350 degrees.
In a medium bowl, combine the beef, oats, milk, egg, onion, salt, and pepper. Use your hands to incorporate everything.
Transfer meat mixture into a loaf pan and press down evenly.
In a small bowl, combine the ketchup, brown sugar, and mustard.
Pour mixture evenly on top of meatloaf.
Place pan into preheated oven and cook for 1 hour and 15 minutes (keep an eye on it as oven temps vary).
Remove from oven and let stand for about 5 minutes before serving! Enjoy!