Preheat the oven to 425 degrees F. Bring a large pot of salted water to a boil over high heat.
Add the fettucine to the boiling water and cook until al dente. Reserve 1 cup of the pasta cooking water, then drain the pasta. Set aside.
Meanwhile, combine the chicken and 2 cups of the mozzarella in a medium bowl. Combine the panko, Parmesan, garlic, oregano, 1 teaspoon salt and a few grinds of black pepper in a separate medium bowl. Pour in the melted butter and mix until well combined.
Add the pasta, chicken mixture, marinara sauce and reserved pasta water to a 9-by-13-inch baking dish and toss until well combined. Sprinkle with the remaining 1 cup mozzarella then top with the panko mixture. Bake until the breadcrumbs on top are golden brown, 25 to 30 minutes. Let cool for 15 minutes. Garnish with basil if desired.