In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter, brown sugar, egg and vanilla until smooth. In a separate bowl, whisk flour, baking soda, cinnamon and salt together. Stir flour mixture into the butter mixture. Stir in the oats, chocolate covered raisins and walnuts.
Drop batter by heaping tablespoons onto the prepared baking sheet, spacing the cookies two inches apart. Place baking sheets in the refrigerator for at least 30 minutes.
Preheat oven to 350°F and place the oven rack in the center of the oven. Bake cookies in preheated oven 10 to 12 minutes, or until the edges are golden brown and the tops look a little undercooked. Allow cookies to sit on the hot baking sheet for five minutes before transferring them to a wire rack to cool.