Preheat oven to 325 degrees. Grease light colored loaf pan (8 1/2" x 4 1/2" x 2 3/4” high) with non-stick cooking spray and set aside.
In a large bowl, mix oil, butter, and sugar until well combined. Stir in eggs, vanilla, and banana until smooth.
Gently stir in dry ingredients and buttermilk until a uniform batter has formed. Scrape sides to ensure no lumps are present. Stir in 1/2 cup walnuts. Pour into prepared pan and top with remaining 1/4 cup chopped nuts.
Bake for 50 to 60 minutes in the lower half of the oven or until completely baked. Remove from oven and cool at least 15 minutes before removing from pan. Slice and serve.