Preheat the oven to 350 degrees F (175 degrees C). Grease a 12-cup muffin tin or line cups with paper liners.

Mix sugar, oil, and egg in a large bowl until light and creamy. Stir in bananas and walnuts. Add flour, baking soda, and salt; stir until completely smooth. Spoon batter into the prepared muffin cups, filling each 3/4 full.

Bake in the preheated oven until tops spring back when lightly pressed, about 30 to 40 minutes.