Stir together flour, 1 1/3 cups oats, salt, and baking soda.
In a large bowl, cream 1/2 cup butter or margarine with 2/3 cup brown sugar. Beat in the eggs, then the bananas and vanilla. Beat the flour mixture into the banana mixture. Turn the batter into a greased and floured 8 inch square pan.
Mix 3/4 cup oats, 1/3 cup brown sugar, melted butter or margarine, walnuts, and cinnamon together until crumbly. Sprinkle evenly over the top of the batter.
Bake in preheated oven at 350 degrees F (175 degrees C) for 40 to 45 minutes, or until it tests done. Transfer to a rack to cool.