Place the oats in a large bowl and cover with water, then cover the bowl with cling film and leave to soak overnight or for at least 4 hrs.
The next day, drain and rinse the oats, then tip them into a blender with 750ml cold water and a pinch of salt. Whizz up until smooth. Pour the mixture into a muslin-lined sieve over a jug and allow it to drip through. Stir the mixture gently with a spoon to speed up the process. Be patient with it as the oats are quite thick.
When most of the liquid has gone through into the jug, gather the sides of the muslin together and squeeze tightly with both hands to extract the last of the milk. Whisk in 50ml cold water to finish. Will keep for two-three days in the fridge.