01 - Preheat oven to 350°F and line two baking sheets with parchment paper.
02 - Whisk together oats, flour, baking soda, cinnamon, and salt in a medium bowl until evenly blended.
03 - Beat softened butter, brown sugar, and granulated sugar in a large bowl until light and fluffy, approximately 2-3 minutes.
04 - Mix in egg and vanilla extract until fully incorporated into the butter mixture.
05 - Gradually fold dry ingredients into wet mixture, mixing until just combined and no dry streaks remain.
06 - Gently fold in raisins or chocolate chips and nuts if desired.
07 - Drop tablespoon-sized dough balls onto prepared baking sheets, leaving 2 inches of space between each cookie.
08 - Bake for 10-12 minutes until edges are golden brown and centers appear set.
09 - Let cookies rest on baking sheet for 5 minutes, then transfer to wire rack to cool completely.