Heat oven to 350°F; parchment-line two sheets. Beat butter and sugar creamy; add egg and vanilla.

Whisk flour, baking soda, cinnamon, and salt; mix into butter. Fold in oats, cranberries, and walnuts.

Stand dough at room temperature 10 minutes while oven finishes heating. Form 24 slightly flattened mounds, 2 inches apart.

Bake one sheet at a time 11 to 12 minutes until edges browned and centers set but yielding.

Cool on sheets 5 minutes; rack-cool fully before sealing in container.