Preheat oven to 350°F (175°C).
Line and grease 9x13 pan with parchment paper.
Mix flour, oats, sugar, salt, baking soda with melted butter; press ⅔ into pan.
Bake base for 15 minutes until lightly golden.
Whisk condensed milk, eggs, and vanilla; fold in white chocolate and cranberries (add walnuts if using).
Spread filling over hot base.
Sprinkle remaining base mixture plus topping white chocolate and cranberries.
Bake 25–30 minutes until golden and set.
Cool completely before cutting; dust with powdered sugar.