A serum is a thin liquid with anti-aging, pore-cleansing ingredients that moisturize skin. Serums for oily skin are applied just before makeup in the morning or a moisturizer at night. Their high concentration of active ingredients can make them more effective than cleansers or lotions. Choosing the right serum is important to ensure your skin remains moisturized without drying out. Serums often contain both synthetic and natural substances like aloe, herbal extracts, and vitamins. A serum for oily skin can have the consistency of a lotion or gel but should never leave your face feeling greasy. Some treat conditions like acne or rosacea while smoothing out wrinkles. If the serum is used underneath makeup or moisturizer, it needs to absorb well and not impede makeup application.