The night before, bring water to boil in a large dutch oven or other heavy bottomed pot. Remove from heat; stir in oats and salt; cover and let sit overnight.
For 1 apple, core and dice; set aside. For 2nd apple, peel, core, and shred on large holes of a box grater. Add shredded apple, milk, cider, brown sugar, and spices to oats.
Bring oat mixture to a boil over medium-high heat, stirring occasionally. Reduce heat to medium-low. Maintain gentle simmer, stirring frequently, until mixture thickens and oats are tender yet chewy; about 10 minutes.
Stir in diced apple and raisins. Continue simmering, stirring frequently, until apples begin to soften, about 5 minutes. Stir in hazelnuts. Adjust flavor and consistency to taste with brown sugar or milk. Serve, topped with yogurt, cream, maple syrup, or fresh fruit.