Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large mixing bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy.
Beat in the pumpkin puree, egg, and vanilla extract until well combined.
In a separate bowl, whisk together the rolled oats, flour, baking soda, cinnamon, nutmeg, and salt.
Gradually add the dry ingredient mixture to the wet ingredients, stirring until just combined. If using, fold in chocolate chips or raisins.
Using a cookie scoop or spoon, drop rounded tablespoons of dough onto the prepared baking sheet, leaving space between each cookie.
Bake in the preheated oven for 10-12 minutes, or until the cookies are lightly golden around the edges.
Allow the cookies to cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.
For added flavor, consider swapping half of the all-purpose flour for whole wheat flour.
These cookies can be stored in an airtight container for up to a week, or they can be frozen for longer storage.