1. Remove top oven rack; move other rack to lowest position. Heat oven to 325°F.
2. Stir together cocoa and hot water in medium bowl; stir in 3 tablespoons sugar and 2 tablespoons oil; set aside. Stir together flour, remaining 1-1/2 cups sugar, baking powder and salt in large bowl; add cold water, remaining 1/2 cup oil and egg yolks; beat with spoon until smooth.
3. Beat egg whites and cream of tartar in another large bowl on high speed of mixer until stiff peaks form. Pour egg yolk mixture in a thin stream over beaten whites, gently folding just until blended. Remove 2 cups batter; add to chocolate mixture, gently folding in until well blended. Fold orange peel into remaining batter.
4. Spoon half of orange batter into ungreased 10-inch tube pan; drop half the chocolate batter on top by spoonfuls. Repeat layers of orange and chocolate batters. Gently swirl with knife for marbled effect, leaving definite orange and chocolate areas.
5. Bake 1 hour and 15 to 20 minutes or until top springs back when touched lightly. Immediately invert cake in pan onto heatproof funnel; cool completely. Remove cake from pan; invert onto serving plate. Prepare ORANGE GLAZE; spread over top of cake, allowing glaze to run down sides. Garnish as desired. Makes 12 servings.
ORANGE GLAZE: Melt 1/3 cup butter or margarine in medium saucepan over low heat. Remove from heat; gradually add 2 cups powdered sugar, 2 to 3 tablespoons orange juice and 1/2 teaspoon freshly grated orange peel, beating with whisk until smooth and of desired consistency. About 1-1/2 cups glaze.