Preheat the oven to 400°F and toast walnuts on a baking sheet for 7-10 minutes. Melt butter in a skillet, toast oats until golden, then transfer to a slow cooker.
Add water, bananas, toasted walnuts, brown sugar, salt, and cinnamon. Cook on low for 4-6 hours. If serving immediately, let it sit for 10 minutes. Store leftovers in the fridge for up to 4 days.