Preheat oven to 350° F, and grease a 9 x 5 inch loaf pan or line with parchment paper.
First prepare the streusel topping. In a small mixing bowl, stir together the sugar, flour, and oats. Pour the melted butter over the dry ingredients and mix well until clumps form. Set the topping aside.
Next, prepare the banana bread batter. In a medium mixing bowl, add the sugar, vegetable oil, and eggs. Whisk until completely smooth and light yellow in color. Add the mashed bananas and vanilla. Mix well.
In separate bowl, stir together the flour, baking soda, and salt. Pour the dry ingredients into the banana mixture, and stir with a spatula just until the flour is incorporated. Do not overmix or it will toughen the bread.
Pour the batter into your greased loaf pan. Top the bread batter evenly with the streusel topping. Immediately place in the preheated oven and bake for approximately 45-50 minutes, or until a tester comes out clean in the center of the bread. Let cool slightly before removing from the pan. Slice and enjoy!