It depends on what you're making. If you're just drinking it as a hot or cold beverage, use 1/4 cup of water and 1/4 cup of oatmilk for every 8 ounces of liquid (so about 1/2 cup total). If you're making something like an oatmeal or porridge, use 1/2 cup of water and 1/2 cup of oatmilk per 8 ounces of liquid (1 cup total).