Preheat oven to 375°F (190°C) and grease a 9x9 inch baking dish.
In a large bowl, whisk together rolled oats, protein powder, baking powder, and salt.
In a separate bowl, whisk together almond milk, eggs, vanilla extract, and Greek yogurt until smooth.
Pour the liquid mixture into the dry ingredients and stir until fully incorporated.
Gently fold in two-thirds of the mixed berries.
Transfer the batter to the baking dish and scatter the remaining berries over the top.
Bake for 35 minutes or until the center is firm and the edges are golden.
Let cool for 5 minutes before serving.