Preheat oven to 350 degrees F (175 degrees C). Line an 8-inch square baking dish with baking parchment.
Spread oats onto a baking sheet and toast in preheated oven until lightly browned, about 10 minutes.
Stir milk, peanut butter, almond butter, and honey together in a saucepan over medium-low heat; cook, stirring regularly, until the nut butters are melted and the mixture is smooth, 2 to 3 minutes. Mash bananas into the milk mixture and cook until smooth, 2 to 3 minutes more.
Remove saucepan from heat and stir toasted oats, protein powder, flaxseed, and sesame seeds into wet ingredients until the mixture is somewhat even; spread into the prepared baking dish.
Bake in preheated oven until lightly browned, about 10 minutes.
Cool bars completely before cutting, wrapping in plastic, and storing in the refrigerator.
