Preheat your oven to 375°F (190°C) to ensure even baking.
Cream together the softened butter, dark brown sugar, and granulated sugar until light and fluffy (2-3 minutes).
Add the eggs one at a time, mixing well after each addition, then add the vanilla extract.
In a separate bowl, whisk together oats, flour, salt, baking soda, and baking powder.
Gradually add the dry ingredients to the wet ingredients, stirring gently until just combined.
Fold in chocolate chips, milk or dark chocolate, and nuts (if using).
Scoop rounded balls of dough onto a parchment-lined baking sheet, spacing them about 2 inches apart.
Bake for about 10-12 minutes until edges are lightly golden.
Let cool on the baking sheet for 5 minutes before transferring to a cooling rack.