Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or lightly grease it.
In a large bowl, mash the bananas until mostly smooth.
Add the vegetable oil, oat milk, and vanilla extract. Stir well to combine. Sprinkle in the flour, sugar, baking soda, and salt. Mix until just combined—don’t overmix!
Fold in the chopped walnuts.
Divide the batter evenly among muffin cups.
Bake for 18–22 minutes, or until a toothpick comes out clean. Let cool for a few minutes before digging in (if you can wait!).