Preheat the oven to 350°F (175°C).

In a large bowl, combine the rolled oats, chopped walnuts, brown sugar, baking powder, ground cinnamon, and sea salt.

In another bowl, mix together the chopped berries and banana pieces.

In a separate bowl, whisk together the milk, egg, olive oil, and vanilla extract.

Combine the wet ingredients with the dry ingredients and mix until just combined.

Gently fold in the berry and banana mixture.

Divide the mixture into greased muffin cups or a greased baking dish.

Bake for 40 minutes, or until the tops are golden and set.

Allow to cool slightly before serving.