Preheat oven to 350 degrees. Lightly spray two baking sheets with oil.
In a mixing bowl, mash banana well. Add oil, sugar and vanilla, and mix with a strong fork.
Add flour, baking soda, salt and cinnamon, and mix until just moistened.
Add oatmeal, walnuts and chocolate chips. Using your hands or a sturdy spoon, mix well, making sure oats are well moistened. (If dough is very slippery, add one or two extra tablespoons flour.)
Using clean, wet hands, re-wetting as needed, roll dough into balls slightly smaller than a golf ball, about 1⅓ inches in diameter. Flatten slightly and place 2 inches apart on a baking sheet.
Bake until lightly browned, 10 to 12 minutes. Remove from heat and allow to cool for 2 minutes, then transfer to a rack to cool completely. Store in an airtight container.