Preheat oven to 180 degrees C. Grease and flour a 20 x 20cm 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 butter and white sugar until light and fluffy. Blend in bananas, eggs and vanilla. Combine the oat flour, plain flour, bicarb 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 centre of the cake comes out clean. Allow to cool.