01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a medium bowl, whisk together oats, flour, baking soda, cinnamon, and salt until well blended.
03 - In a large bowl, beat butter and brown sugar until light and fluffy. Add maple syrup, egg, and vanilla, mixing until smooth and creamy.
04 - Gradually add dry ingredients to wet ingredients, mixing just until combined. Do not overmix.
05 - Gently fold in raisins, diced apple, and walnuts if using until evenly distributed throughout the dough.
06 - Drop rounded tablespoonfuls of dough onto prepared baking sheets, spacing about 2 inches apart to allow for spreading.
07 - Bake for 11-13 minutes, or until cookies are golden brown around the edges and set in the center.
08 - Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely before storing.