Heat oven to 350°F (or 325°F for dark or nonstick pan). Reserve 1/2 cup cake mix for topping. In large bowl, beat remaining cake mix, 1/2 cup butter and the eggs with electric mixer on medium speed until dough forms. Stir in chocolate chips. Spread in ungreased 13x9-inch pan. Bake 14 to 18 minutes or until set.
Meanwhile, in 2-quart saucepan, heat caramels, 1/4 cup butter and the milk over medium heat about 8 minutes, stirring frequently, until caramels are melted and mixture is smooth.
Spread caramel filling evenly over partially baked crust. In small bowl, mix reserved 1/2 cup cake mix, the oats and 3 tablespoons butter with fork until crumbly. Sprinkle over caramel filling.
Bake 18 to 22 minutes longer or until top is set. Cool completely, about 2 hours. Run knife around sides of pan to loosen bars. For bars, cut into 8 rows by 6 rows.