Mix oatmeal, sugar, flour, cinnamon, nutmeg, and salt in a small bowl.
Stir melted butter and vanilla into mixture until it is crumbly.
Put half of the sliced apples in Crock Pot and sprinkle half of oatmeal mixture on top.
Cover with remaining apples and top with the rest of the oatmeal mixture.
Cook on high for 2 to 2.5 hours.