Stir together oats, raisins, walnuts, brown sugar, and cinnamon in a bowl. Pour 1/3 cup oatmeal mixture into each of 8 small zip-top storage bags and store airtight.
To cook, empty 1 bag into a deep microwavable bowl. Stir in 2/3 cup water. Microwave on high for 2 minutes. Stir again.