Preheat oven to 350°F. In a large bowl, combine ground beef, oats, egg, ¼ cup ketchup, onion, garlic, salt, and pepper. Mix with your hands until just combined—don't overmix!
Shape the mixture into a loaf and place in a greased 9x5-inch loaf pan. Spread remaining ¼ cup ketchup on top.
Bake for 1 hour or until internal temperature reaches 160°F. Let rest for 5 minutes before slicing.
While meatloaf bakes, place potatoes in a large pot and cover with cold salted water. Bring to a boil and cook for 15-20 minutes until fork-tender.
Drain potatoes and return to pot. Add butter and milk, then mash until smooth and creamy. Season with salt and pepper to taste.
Slice meatloaf and serve alongside mashed potatoes. Drizzle any pan drippings over the top for extra flavor!