Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a large mixing bowl, combine the rolled oats, flour, brown sugar, granulated sugar, baking soda, ground cinnamon, and salt. Whisk until well blended.
In another bowl, cream the softened butter until smooth. Add the egg and vanilla extract, and mix until fully incorporated.
Gradually add the wet mixture to the dry ingredients, stirring until just combined.
Fold in the diced apple, chopped nuts (if using), and dried cranberries or raisins (if using) until evenly distributed throughout the dough.
Using a tablespoon or cookie scoop, drop rounded balls of dough onto the prepared baking sheet, spacing them about 2 inches apart.
Bake in the preheated oven for 12-15 minutes or until the edges are golden brown. The centers may appear slightly soft; they'll firm up as they cool.
Remove from the oven and allow to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
Notes
Optional ingredients can be adjusted based on preference.