Blend the oats, seeds, milk and banana if using in a high speed blender or with an immersion blender until smooth.
Pour the mixture into a tall jar and stir through the yoghurt using a wooden spoon.
Cover the jar with a fine kitchen towel or cheese cloth and secure with a rubber band.
Let the oats rest in a dry and warm place (ideally about 25C). I like using themoven with the lights and temperature lightly turned on. You could also put it onto your heater or let it sit on the counter top, depending on how warm it is where you live.
The fermentation time should last for at least 8 hours. By the time the oats are ready, they should have built some bubbles.
Let cool in the fridge where it will become more creamy and thicker again. Enjoy with your favourite toppings!