Preheat oven to 400 degrees Fahrenheit.
Combine breadcrumbs and milk in a bowl and allow to hydrate for 5 minutes.
Mix ground beef, ground pork, egg, Parmesan, garlic, oregano, salt, and pepper into the breadcrumb mixture until homogenous.
Shape the mixture into 12 uniform meatballs and place on a lined baking sheet.
Bake meatballs for 18 minutes or until internal temperature reaches 165 degrees Fahrenheit.
Transfer meatballs to a saucepan containing marinara sauce and simmer over low heat for 10 minutes.
Halve hoagie rolls, apply butter, and toast under a broiler for 1 to 2 minutes.
Assemble sandwiches by placing 3 meatballs in each roll, covering with extra sauce, and topping with 2 slices of provolone.
Broil assembled sandwiches for 2 minutes until cheese is melted and bubbling.