Preheat oven to 350°. Combine first 4 ingredients in bowl of a heavy-duty electric stand mixer. Make a well in center of flour mixture; add oil, egg yolks, and orange juice. Beat at medium-high speed 3 to 4 minutes or until smooth. Stir in zest.
Beat egg whites and cream of tartar at medium-high speed until stiff peaks form. Gently fold into flour mixture. Spoon batter into 3 greased and floured 9-inch round cake pans. Bake at 350° for 17 to 20 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes; remove from pans to wire racks, and cool completely (about 1 hour). Spread Lemon-Orange Buttercream Frosting between layers and on top and sides of cake.
Beat butter, orange zest, and lemon zest at medium speed with an electric mixer 1 to 2 minutes or until creamy; gradually add powdered sugar alternately with lemon juice and 5 Tbsp. fresh orange juice, beating at low speed until blended after each addition. Add up to 1 Tbsp. additional fresh orange juice, 1 tsp. at a time, until desired consistency is reached.