Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper if desired.
In a mixing bowl, cream together the softened butter, brown sugar, and granulated sugar until smooth and fluffy.
Beat in the egg and vanilla extract until well combined.
In another bowl, whisk together the rolled oats, flour, baking soda, cinnamon, and salt.
Gradually add the dry ingredients to the wet mixture, stirring until just combined. If using, fold in the chocolate chips or raisins.
Drop tablespoon-sized portions of dough onto the prepared baking sheet, spacing them about 2 inches apart.
Bake in the preheated oven for 10 to 12 minutes, or until the edges are golden brown.
Let the cookies cool on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely.