Preheat oven to 350 degrees and lightly grease an 8 x 8 baking dish or line with parchment paper. Set aside.
Combine oats, flour, salt and baking powder in a bowl and stir to thoroughly combine.
In a large bowl stir together bananas, nut butter, maple syrup and vanilla.
Add a third of the flour mixture to wet ingredients and fold together. Continue adding flour and gently stir together until all flour has been added. Batter will be thick.
Add walnuts, if using and chocolate chips and gently fold in.
Spread batter in prepared baking dish in an even layer.
Bake for 18-22 minutes or until lightly browned and a toothpick inserted comes out clean.
Allow to cool completely.
To prepare chocolate drizzle. Combine chocolate and oil in a microwave safe dish and warm in 15 second intervals stirring between each until chocolate has melted.
Drizzle chocolate over cooled oatmeal bars and place the pan in the fridge for 20 minutes or until chocolate has set and hardened. After set cut into desired number of squares.