Preheat oven to 180 degrees C. Grease and flour a 20x30cm pan. Squeeze orange and reserve 1/3 cup juice. Grind orange peel and pulp, raisins and 1/3 cup walnuts together. Set aside.
In a large bowl, blend flour, 1 cup sugar, bicarb soda, salt, milk, copha and eggs. Beat 3 minutes at medium speed. Stir in orange-raisin mixture. Pour into prepared pan.
Bake in the preheated oven for 35 to 40 minutes, or until a toothpick inserted into the centre of the cake comes out clean. Drizzle reserved orange juice over warm cake. Combine 1/3 cup sugar, cinnamon and 1/4 cup walnuts; sprinkle over cake. Garnish with orange slices.