Preheat oven to 375°F. Line a baking sheet with parchment paper or lightly spray with cooking spray.
In a large bowl, combine ground beef, oats, onion, eggs, milk, garlic, Italian seasoning, salt, pepper, and ¼ cup ketchup. Mix gently with your hands until just combined—don't overmix or the meatloaf will be tough.
Shape mixture into a loaf (about 9x5 inches) on the prepared baking sheet.
In a small bowl, whisk together remaining ¼ cup ketchup, brown sugar, and apple cider vinegar to make the glaze.
Brush half of the glaze over the top of the meatloaf.
Bake for 35 minutes, then brush with remaining glaze. Continue baking for 10 more minutes, or until internal temperature reaches 160°F.
Let rest for 5 minutes before slicing. Cut into 6 equal slices and serve warm.