Preheat the oven to 350 degrees F (175 degrees C). Grease and flour a 9x13-inch baking pan.

Beat butter, brown sugar, and white sugar together in a large bowl until thoroughly combined and creamy. Beat in eggs, one at a time, mixing well after each addition. Stir in vanilla extract until combined. Stir in rolled oats, flour, chocolate chips, raisins, walnuts, baking soda, baking powder, and salt until all ingredients moistened; spread into the prepared baking pan.

Bake in the preheated oven until golden brown, 20 to 25 minutes. Let cool in the pan for about 5 minutes before cutting into bars.