1. Preheat the oven to 350 degrees F and grease a 10-inch tube pan (the kind that leaves a hole in the middle).
2. Peel and core the apples and chop them into smallish pieces. Toss them with the cinnamon and 5 tbsp. of the sugar, and set aside.
3. In a large bowl, combine the flour, baking powder and salt. In another bowl, add the oil, orange juice, vanilla, eggs and remaining sugar. Whisk. Slowly add the wet ingredients to the dry ones, stirring constantly. Use your spoon or a spatula to scrape the edges of the bowl and make sure everything is mixed and smooth. If you want to use the walnuts, add them now.
4. Pour half of the batter into the pan. Add about half of the apple pieces, spreading them out evenly. Pour the rest of the batter into the pan, and spread the remaining apple pieces on top.
5. Bake for about 90 minutes, or until a toothpick comes out clean. Cool completely. To remove it from the pan, run a knife around the outer and inner edges of the cake. Set a plate, serving side down, on top of the cake, hold everything in place and flip it. Sprinkle with icing sugar and serve.