Preheat the oven to 375°F. Line a muffin pan with paper liners or grease the cups with nonstick spray.
In a large bowl, combine 2 cups rolled oats, 1 teaspoon baking powder, 1/2 teaspoon ground cinnamon, and 1/4 teaspoon sea salt.
In a separate bowl, mash 2 ripe bananas with a fork until mostly smooth. Whisk in 1 1/2 cups milk, 1 large egg, and 1/4 cup maple syrup until blended.
Pour the wet ingredients into the dry ingredients and stir until just combined (avoid overmixing).
Gently fold in 1 cup mixed berries (chop strawberries if using) until evenly distributed.
Divide the batter evenly among the prepared muffin cups, filling each about 3/4 full.
Bake for 25 minutes, or until the tops are golden and a toothpick inserted into the center comes out clean.
Let the oatmeal cups cool briefly in the pan, then transfer to a wire rack to cool before serving.