Preheat oven to 350°. Place oats and butter in bowl of an electric mixer. Pour 1 1/4 cups boiling water over oat mixture; cover and let stand 20 minutes.
Stir together flour, baking soda, cinnamon, and nutmeg.
Add eggs, 1 at a time, to oat mixture, beating at medium speed just until blended after each addition. Gradually add sugars, beating just until blended. Gradually add flour mixture, beating just until blended. Pour batter into a greased and floured 13- x 9-inch pan.
Bake for 26 to 30 minutes or until a wooden pick inserted in center of cake comes out clean. (Cake will begin to pull away from sides of pan.) Remove cake from oven; increase oven temperature to broil.
Spoon Brown Sugar-Pecan or Brown Sugar-Walnut Topping onto warm cake. Gently spread topping over cake using back of a spoon.
Broil cake 3 inches from heat 3 to 5 minutes or until sugar is bubbly and lightly browned. Let cake cool completely (about 3 to 4 hours).