Add the frozen berries, frozen banana, frozen acai, and oat milk to a high speed blender.
Start blending on low speed and gradually work up to high speed, scraping the sides of the blender as needed with a tamper tool. Don't have a tamper tool? Stop the blender and just use a rubber spatula. The mixture should be thick.
Blend until the mixture is completely smooth, about 45-60 seconds.
Transfer the acai mixture to a bowl and top with your desired toppings.