Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper.
In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
In a separate bowl, whisk together the old-fashioned oats, all-purpose flour, all purpose flour, baking powder, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the creamy peanut butter until evenly distributed throughout the cookie dough.
Gently fold in the M&M's or Smarties, chocolate chips, and chopped nuts if using.
Drop rounded tablespoons of dough onto the prepared baking sheets, leaving enough space between each for spreading.
Bake in the preheated oven for 10-12 minutes or until the edges are lightly golden but the centers are still soft.
Allow the cookies to cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.
