Preheat the oven to 350 degrees F (175 degrees C).

Mix ground beef, quick oats, onion, milk, eggs, 1/4 cup ketchup, parsley, garlic, salt, basil, and black pepper together in a large bowl until thoroughly combined; pat the mixture into a 9x5-inch loaf pan.

Bake meatloaf in the preheated oven until firm and no longer pink inside, 1 to 1 1/4 hours. An instant-read meat thermometer inserted into the center of the meatloaf should read at least 160 degrees F (70 degrees C).

Remove from oven, spread 2 tablespoons ketchup over loaf, and top with Cheddar cheese slices; return to oven and bake until cheese has melted, about 5 more minutes.