Preheat the oven to 350 degrees F (175 degrees C).
Combine flour, oats, baking soda, and salt in a bowl.
Beat butter in a bowl using an electric mixer for 30 seconds. Add sugar and beat until fluffy. Beat in the oat mixture until crumbly; do not overmix. Reserve 1 cup of the crust mixture and press the rest into the bottom of a 9x13-inch rimmed baking pan.
Bake in the preheated oven until firm and browned, about 10 minutes. Leave oven on; remove crust and let cool in the pan for 10 minutes.
Combine caramels and evaporated milk in a saucepan over low heat. Cook and stir until caramels melt and mixture is smooth, about 5 minutes. Spread over the prebaked cookie crust; sprinkle reserved crust mixture, nuts, and butterscotch chips on top.
Bake in the hot oven until golden, 15 to 20 minutes. Cool in the pan on a wire rack. Cut into bars.