Prepare the apples. Peel, core, and chop apples into bite-sized chunks. Toss with cinnamon and sugar, then set aside.
Mix dry ingredients. In a large mixing bowl, sift together the flour, baking powder, and salt.
Mix wet ingredients. In a separate bowl, whisk together oil, sugar, orange juice, and vanilla. Add eggs one at a time, whisking until smooth.
Combine batter. Stir wet ingredients into the dry mixture until just combined. Fold in walnuts if using.
Assemble the cake. Grease a bundt or tube pan. Pour half the batter into the pan, spread half the apple mixture over it, then repeat with remaining batter and apples.
Bake. Bake at 350°F for 1 ½ hours, or until a toothpick inserted in the center comes out clean.
Cool and serve. Let cool for at least 20 minutes before removing from pan. Serve warm or at room temperature.