Yes, you can make the base of overnight oats in bulk. Just keep the quantity under what you can finish off within 5 days. Also, if you are making something to consume after more than a day mix only the oats, milk, chia seeds, and sweetener. It's best to add the fruits maximum a day in advance. With time, the oats may become thicker in the fridge, just add more milk to get the desired consistency before serving.