1. Preheat oven to 350°.
2. Combine sugar and butter in a large bowl. Stir in vanilla and egg. Weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour, oats, 1/2 teaspoon salt, and baking soda in a bowl. Add oat mixture to sugar mixture; stir with a fork until combined (mixture will be crumbly). Place 3 cups oat mixture into the bottom of a 13 x 9-inch baking pan coated with cooking spray; press into bottom of pan. Set aside.
3. Place sweetened condensed milk, butterscotch morsels, and 1/8 teaspoon salt in a microwave-safe bowl; microwave at HIGH 1 minute or until butterscotch morsels melt, stirring every 20 seconds. Stir in walnuts. Scrape mixture into pan, spreading evenly over crust. Sprinkle evenly with remaining oat mixture, gently pressing into butterscotch mixture. Bake at 350° for 30 minutes or until the topping is golden brown. Place pan on a cooling rack; run a knife around outside edge. Cool completely.