Heat olive oil in a small skillet over medium heat. Add onions and cook for 5 minutes. Add garlic and cook for 1 more minute. Set aside and let cool.
Combine the breadcrumbs, Parmesan cheese, oregano, salt, and pepper and set aside.
Crack an egg in a small bowl and whisk it. Add the milk and whisk to combine.
Combine the dry mixture with the wet mixture to form a smooth paste. Mix in the onions and garlic. (This is called a panade and keeps the juiciness and flavor intact while the meatballs cook.)
Add the meat to a large bowl and top with the panade. Use your hands and gently mix until just combined. Don’t overwork the meat or the meatballs will be tough.
Roll into 1 ½ inch meatballs.
Pour ¾ of the marinara sauce on the bottom of a 9×13 inch casserole dish. Add the meatballs. Spoon remaining sauce over the top.
Cover and bake for 30 minutes.
Uncover and top with cheese. Bake 15 more minutes. Broil for additional 1-2 minutes at the end if desired. Sprinkle with parsley and serve.