Preheat oven to 375°F (190°C) and line a baking sheet with parchment paper.
Cook spaghetti in a large pot of salted boiling water according to package directions until al dente; drain and return to pot.
Incorporate marinara sauce with the cooked spaghetti until evenly coated.
Using a serrated knife, cut a rectangular trough out of the top center of the bread loaf, leaving a 1-inch border on all sides and bottom.
In small bowl, whisk together melted butter, minced garlic, oregano, and salt.
Brush the interior cavity of the bread with approximately half of the garlic butter mixture.
Transfer the sauced spaghetti into the bread cavity, compressing slightly to fit.
Distribute the mozzarella and parmesan cheeses evenly over the spaghetti filling.
Brush the exposed crust of the bread with the remaining garlic butter.
Bake for 15 to 20 minutes until the bread is crisp and the cheese is melted and golden brown.