Mash 3/4 of banana slices in a bowl; stir in egg whites and oats to form a batter.
Spray a skillet with cooking spray. Drop batter by large spoonfuls into the skillet and cook until browned, 3 to 4 minutes. Flip and cook until browned on the other side, 2 to 3 minutes. Repeat with remaining batter.
Place oat cakes on a serving plate. Top with peanut butter and remaining banana slices.