In a mixing bowl, combine cooked chicken, refried beans, chili powder, and oregano. Mix well. Spoon chicken mixture into uncooked manicotti shells. Place onto a prepared 9"x13" pan; set aside. If any filling remains, use in taco shells. In a small bowl, combine water and picante sauce. Pour over shells in pan. Cover and refrigerate overnight. Preheat oven to 350. Prior to baking, remove manicotti from refrigerator 30 minutes before. Cover and bake for one hour. Uncover and spread sour cream evenly over top. Sprinkle with cheese. Bake for ten minutes more or until cheese has melted.