In a large mixing bowl, stir together the banana, oil, sugar, and vanilla extract.
Add the flour, baking soda, salt, and cinnamon. Stir just until combined.
Stir in oats, walnuts, and chocolate chips.
Working in 2 batches, roll the dough into 1 1/2-inch balls and place at least 2 inches apart onto a baking sheet.
Bake for 10-12 minutes, until lightly browned.
Transfer the cookies to a cooling rack. Allow to cool completely then serve.
Notes
1. Pecans, almonds, or cashews are all good substitutions for walnuts.
2. To make these cookies nut-free - Just swap the walnuts for an equal amount of dried cranberries or other small dried fruit.