Preheat the oven to 350 degrees F (175 degrees C).

Combine flour, oats, brown sugar, and butter in a large bowl. Set 1/2 cup of the mixture aside. Press remaining mixture over the bottom of a 9x13-inch glass baking dish.

Bake in the preheated oven until firm, about 10 minutes.

Pour sweetened condensed milk over the crust. Sprinkle cranberries, walnuts, chocolate chips, and orange zest over the milk. Top with the reserved oat mixture; press down lightly.

Continue baking until lightly browned, about 25 minutes. Cool completely before cutting into 12 bars.