Preheat your oven to 350°F (175°C).
Grease a 12-cup muffin tin with cooking spray or line with paper liners.
In a large mixing bowl, mix in the dry ingredients by combining the rolled oats, baking powder, ground cinnamon, and salt.
Stir well!
In a separate bowl, whisk together the wet ingredients by combining the unsweetened almond milk, eggs, maple syrup or honey, and vanilla extract until well combined.
Combine wet and dry ingredients. Pour the wet ingredients into the bowl with the dry ingredients and mix until everything is well incorporated.
Add berries and optional add-ins now.
Gently fold in the mixed berries and any optional add-ins (such as chopped nuts or chocolate chips) until evenly distributed throughout the oatmeal mixture.
Divide the oatmeal mixture evenly among the prepared muffin cups, filling each cup to the top.
Place the muffin tin in the preheated oven and bake for 20-25 minutes, or until the oatmeal cups are set and lightly golden on top.
Once baked, remove the oatmeal cups from the oven and let them cool in the muffin tin for a few minutes.
Transfer the oatmeal cups to a wire rack to cool completely.
Serve the oatmeal cups warm or at room temperature.
They can be enjoyed plain or topped with a dollop of Greek yogurt, a drizzle of maple syrup, or additional fresh berries if desired.