Preheat the oven to 375° F. In an 8x11 inch baking dish, combine the oats, cinnamon, baking powder and salt.
In a large mixing bowl, mash the bananas, then add the almond milk, peanut butter, maple syrup, flaxseed and vanilla extract. Allow the mixture to stand for 5 minutes for the flaxseed to set.
Pour over the wet ingredients over the oat mixture and stir to combine.
Bake uncovered in the preheated oven until the top of the oatmeal is golden and the mixture is set, about 30-35 minutes. Remove and allow it to cool for 5 minutes.
Serve with drizzled peanut butter and banana slices, if desired.