Preheat your oven to 350°F (175°C) to ensure even cooking.
Lightly grease your muffin tin with cooking spray or line it with muffin liners for easy removal.
In a mixing bowl, combine the rolled oats, baking powder, ground cinnamon, and sea salt. Mix well to distribute the baking powder evenly.
In another bowl, mash the ripe bananas until smooth. Add the milk, egg, and maple syrup, then whisk until combined.
Pour the wet mixture into the dry ingredients bowl. Stir until just combined, being careful not to over-mix; a few lumps are fine.
Gently fold in the mixed berries, ensuring they are evenly distributed throughout the mixture.
Using a spoon, evenly distribute the oatmeal mixture into the prepared muffin tin, filling each cup about three-quarters full.
Bake in the preheated oven for 18-20 minutes, or until the tops are golden and a toothpick inserted in the center comes out clean.
Remove the Oatmeal Cups from the oven and allow them to cool in the pan for about 5 minutes before transferring to a wire rack to cool completely.