Combine oats, sugar and butter, mixing until crumbly. Place in a shallow baking pan. Heat in preheated moderate oven (350 F.) about 10 minutes, stirring occasionally. Cool 15 minutes, then toss lightly with a fork to form crumbs. Stir in peanuts. Store in covered jar in refrigerator. Stir with fork before using. Serve as a topping on sundaes, puddings and other desserts.