Preheat your oven to 350°F. Grease a 9×13-inch baking dish.
In a large bowl, combine the shredded chicken, ricotta cheese, beaten egg, 1 cup of the mozzarella cheese, the grated Parmesan, dried oregano, salt, and pepper. Mix until well blended.
Fill each cooked manicotti shell with the chicken and cheese mixture using a piping bag or a plastic bag with a corner cut off. Take care not to overfill to prevent breakage.
Arrange the stuffed shells in a single layer within the prepared baking dish.
Combine the marinara sauce, water, and Italian seasoning, then pour the mixture evenly over the shells.
Sprinkle the remaining ¾ cup of mozzarella cheese over the top. If desired, sprinkle with extra Parmesan for a crispier top.
Bake in the preheated oven for 35-40 minutes, or until the sauce is bubbling and the cheese has melted.
Remove from oven and let stand for 5 minutes before serving. Garnish with fresh basil if desired.
