Heat the oven to 350°F and line two baking sheets with parchment. Beat butter and brown sugar until creamy, then beat in egg and vanilla.
Whisk flour, baking soda, cinnamon and salt. Mix into the butter, then fold in oats, cranberries and walnuts.
Let the dough stand at room temperature for 10 minutes while the oven finishes heating. Divide into 24 portions and place 2 inches apart, flattening the mounds slightly.
Bake one sheet at a time for 11 to 12 minutes, until the edges are browned and the centers are set but still yielding.
Cool on the sheets for 5 minutes before moving to racks, and cool fully before sealing in a container.