In a small bowl, combine water and oats; let stand for 10 minutes. Meanwhile, in a large bowl, cream butter and sugars until light and fluffy. Add the eggs, one at a time, beating well after each addition. Beat in oat mixture. Combine the flour, cocoa, baking soda and salt; gradually add to creamed mixture and mix well. Stir in 1 cup chips.
Pour into a greased 13-in. x 9-in. baking pan. Sprinkle with walnuts and remaining chips. Bake at 350° for 40-45 minutes or until a toothpick comes out clean. Cool on a wire rack.