Preheat the oven to 350°F (175°C) and grease a 9x13-inch baking dish.
In a large bowl, combine the shredded cooked chicken, ricotta, beaten egg, 1 cup of the shredded mozzarella, 1/3 cup grated Parmesan, dried oregano, and salt and pepper; mix until well blended.
Transfer the filling to a piping bag or a resealable plastic bag with a corner snipped off and fill each cooked manicotti shell, taking care not to overfill to avoid tearing.
Arrange the stuffed manicotti in a single layer in the prepared baking dish.
Stir the marinara sauce with 1/2 cup water and the Italian seasoning, then pour the sauce evenly over the shells.
Sprinkle the remaining 3/4 cup shredded mozzarella over the sauce and add extra Parmesan on top if desired.
Bake for 35–40 minutes, until the sauce is bubbling and the cheese is melted and lightly golden.
Remove from the oven and let rest 5 minutes, then garnish with chopped fresh basil if using and serve.