Preheat oven to 350 degrees F. Grease 9-inch cake pan with cooking spray or butter.
Make the crumble. Melt ½ cup butter and let cool. Combine the butter with ¾ cup oats, ½ cup brown sugar, ½ cup chopped pecans, and ¼ cup flour. Stir to combine and set aside.
Combine dry ingredients including flour, baking powder, baking soda, salt, cinnamon, and nutmeg in a large bowl.
Make the cake. In a separate bowl, cream together butter and sugar until fluffy and well combined, about 3 minutes. Add egg, eggnog, and vanilla and beat until combined. Add the dry ingredient mix, and stir just until combined.
Pour the batter into the pan. Sprinkle the crumble in an even layer on the batter. Bake for about 45 minutes - 1 hour or until a wooden pick inserted in the center comes out clean. Serve warm or at room temperature.