Combine ground beef, ground turkey, egg, celery, rolled oats, skim milk, ketchup, onion, green pepper, parsley, dry mustard, salt, minced garlic, and black pepper in a bowl until thoroughly combined; mixture will be moist.
Transfer mixture into a 5x9-inch loaf pan.
Bake in the preheated oven until an instant-read meat thermometer inserted into the center of the meatloaf reads at least 160 degrees F (70 degrees C), about 1 1/2 hours. Meatloaf may still look pink inside due to ketchup.
