Preheat the oven to 375°F. Line a baking sheet with parchment and set aside.
In a large bowl or stand mixer, add the butter, brown sugar and granulated sugar. Beat until creamy and fluffy.
Add the egg and vanilla extract. Beat until incorporated and smooth with a hand mixer.
Add the flour, baking soda, salt and cinnamon. Mix until incorporated.
Add the oats and optional raisins/walnuts, mixing until incorporated.
Add tablespoonfuls or use a 1 ½-inch cookie scoop to drop batter onto the prepared baking sheet, spaced 2 inches apart to allow for expansion.
Chill the uncooked cookie balls for 30 minutes in the refridgerator.
Bake for 7-10 minutes or until the edges start to turn golden brown and the centers still look under-baked.
Remove the baking sheet from the oven. Using a rigid spatula, transfer the cookies to a wire cooling rack.
Cool slightly before serving. To store for later, cool completely and place in airtight containers.