Preheat oven to 350 degrees F (175 degrees C). Grease and flour an 8x8 inch pan. Make the oat crunch topping: In a small bowl, combine the rolled oats, brown sugar, melted butter and 2 tablespoons chopped walnuts.
Mix well and set aside.
In a large bowl, cream shortening and white sugar until light and fluffy. Blend in bananas, eggs, and vanilla.
Combine the oat flour, all-purpose flour, baking soda and salt. Beat into creamed mixture. Stir in 1/2 cup chopped walnuts.
Pour batter into the prepared pan.
Sprinkle with the oat crunch topping.
Bake in the preheated oven for 35 to 40 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool.