Prepare the baking dish and oven:
Preheat your oven to 375°F. Grease a 9x9-inch baking dish thoroughly with butter or line it with parchment paper to prevent sticking.
Mix the ingredients:
In a large mixing bowl, combine the rolled oats, milk, eggs, melted butter, honey, yogurt, vanilla extract, and salt. Stir the mixture until the ingredients are well incorporated and form a cohesive batter.
Incorporate the berries:
Gently fold 3/4 cup of blueberries and 3/4 cup of raspberries into the oatmeal mixture. Pour the prepared mixture into the greased or lined baking dish, ensuring an even distribution. Scatter the remaining berries across the surface for a visually appealing topping.
Bake the casserole:
Transfer the dish to the preheated oven and bake for approximately 45 minutes. The oatmeal is ready when the surface appears glossy and firm to the touch. Allow it to cool slightly before serving.