While the butter and sugar are being creamed, whisk the flour, baking soda, cinnamon, and salt together in a small bowl. Add to the butter and sugar mixture.
In 2 batches, add the oats to the dough. Then add the raisins and chopped walnuts, stirring until everything is combined.
Scoop the dough onto a cookie sheet or plate. Chill for at least 30 minutes, and up to overnight.
When it comes time to bake the cookies, line two baking sheets with parchment paper and set aside. Preheat the oven to 350 degrees F.
Place the dough on the prepared baking sheets, about 2 inches apart. Bake for 10-12 minutes, or until the edges are golden but the centers are slightly underdone looking.