Finely grate the zest from half of the orange into a medium bowl. Stir in the milk, vanilla, ¼ teaspoon of the cinnamon, salt, oats, and cranberries. Cover and refrigerate overnight. Save the remaining orange.
In the morning, divide the mixture among four bowls and top each evenly with the remaining ¼ teaspoon of cinnamon, walnuts, and maple syrup. Cut off the rind from the outside of the reserved orange, following the contour of the fruit. Cut V shapes between the membranes to release the segments. Chop the segments and scatter over each bowl.
Tip: If you’re avoiding dairy, you can substitute almond, coconut, or soy milk. If you like chewier oatmeal, use steel-cut oats and 2 ¼ cups milk.