Preheat your oven to 180°C (350°F).
Line an 8×8-inch baking pan with parchment paper to prevent sticking.
Set the pan aside while you prepare the batter.

In a large mixing bowl, mash the ripe bananas with a fork until mostly smooth.
Add the rolled oats and almond butter to the bowl and mix everything together.
If you prefer sweeter oatmeal, stir in the honey or maple syrup at this stage.

Fold in half of the blueberries using a rubber spatula.
Mix gently to avoid crushing them too much.
If the batter feels too thick, add a little milk to loosen it.

Transfer the batter into the prepared baking pan.
Spread it out evenly using the back of a spoon or spatula.
Top the batter with the remaining blueberries and sprinkle with nuts if using.

Bake in the preheated oven for 15–20 minutes.
The edges should be golden brown, and a toothpick inserted into the center should come out mostly clean.
Avoid overbaking to keep the oatmeal moist.

Remove the baked oatmeal from the oven and allow it to cool in the pan for 15 minutes.
Carefully lift it out using the parchment paper and place it on a wire rack to cool completely.
Slice into squares and enjoy warm or at room temperature.