Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.

Mix oats, 3/4 cup flour, brown sugar, baking soda, and 1/4 teaspoon salt together in a bowl. Stir 3/4 cup melted butter into oat mixture until evenly coated. Measure 3/4 cup oat mixture and reserve for later. Press remaining oat mixture into the prepared baking dish.

Bake in the preheated oven until lightly browned, about 5 minutes.

Combine white sugar, 1 cup melted butter, and vanilla extract in a bowl. Beat eggs, 1 at a time, into sugar mixture, mixing well after each addition, until thoroughly blended.

Sift 1 1/2 cups flour, cocoa powder, and 1 teaspoon salt together in a bowl; stir into sugar-egg mixture just until batter is blended. Fold chocolate chips into batter; pour over oat crust. Sprinkle reserved oat mixture over batter.

Bake in the preheated oven until a toothpick inserted in the center of the brownies comes out clean, 35 to 40 minutes.