By adding an oil to your skincare routine, even if you have oily skin, you will be helping yourself. Not moisturizing already oily skin might mean your skin will make more oil. Oil is produced in the body when it senses that something is dry, therefore adding a layer of oil can send a message to your skin to stop making as much oil because it's already hydrated. For oily skin, add an oil at night and let it soak overnight.