In a medium bowl, mix together the oats, brown sugar, flour, nutmeg, cinnamon, and salt.
Add the cubed butter to the bowl. Use a pastry cutter or your fingers to blend the butter into the dry ingredients until the mixture resembles coarse crumbs.
To bake, sprinkle the crumble topping evenly over your dessert and bake as directed in your recipe. If baking separately, spread on a parchment-lined baking sheet and bake at 350°F for about 12-15 minutes, or until golden brown.