Preheat oven to 350°F and line the bottom of an 8 x 8 inch glass dish with parchment paper. (We also used our fingers to grease the sides of the dish with coconut oil.)
Mash the bananas and mix together with oats and peanut butter.
Spread the dough evenly in the prepared dish, then sprinkle with walnuts and chocolate chips. Lightly press the nuts and chocolate into the bars.
Bake for 18-20 minutes or until an inserted toothpick comes out clean. Cool for 20 minutes in the pan, then slide a spatula under the parchment paper and carefully transfer the whole uncut bar onto a rack for further cooling. (You can cut the bars at this point, and eat them warm! The more time you give them to cool, the easier it will be to cut them.)