Preheat your oven to 350°F (175°C). In a medium bowl, combine the rolled oats, all-purpose flour, baking soda, ground cinnamon, and salt. In a separate large mixing bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy. Add the egg and vanilla extract, then gradually mix in the dry ingredients. Fold in the diced apples, nuts, and raisins or cranberries if using. Drop tablespoon-sized portions onto a lined baking sheet, spacing them 2 inches apart. Bake for 10-12 minutes until edges are golden and centers are soft. Let cool on the baking sheet for 5 minutes before transferring to a wire rack.