Preheat the oven to 350ºF. Coat an 8" x 8" baking pan with cooking spray.
Spread the walnuts on an ungreased baking sheet and toast in the oven for 5 to 8 minutes, or just until fragrant.
Meanwhile, mash the bananas in a medium mixing bowl. Add the oil, sugar, and vanilla extract. Mix until smooth.
In a large mixing bowl, combine the oats, flour, baking powder, salt, cinnamon, nutmeg, and baking soda. Add the banana mixture and stir until just combined. Fold in the walnuts and cranberries, being careful not to overmix.
Pour the mixture into the prepared pan and spread evenly. Bake for 25 to 30 minutes, or until the top is browned and a toothpick inserted in the middle comes out nearly clean. Allow to cool completely before cutting.