Combine the oats, brown sugar, salt, cinnamon and bananas with 5 1/2 cups water in the slow cooker. Stir to mix evenly.
Cover with a lid and cook on low until the oats soften, 6 to 8 hours. Before serving, remove the lid and let the oatmeal cool slightly before stirring. Adjust seasonings, as needed.
Serve in bowls topped with fresh berries and banana slices.