When applying products to skin always go with the product with the thinnest and most watery consistency first — serum first, oil after.
Serums deliver a higher concentrate of working ingredients deeper into the skin. They are made up of tiny particles with active elements. This creates more hydration deep within the skin instead of heavier products or creams that may sit on top of the skin.
Oils have a thicker consistency and are lipophilic. Like serums, they can penetrate deeper into the skin. This helps lock in moisturizing ingredients while acting as a barrier to keep germs or harmful toxins out.
Keep in mind that serums are generally used to treat particular skincare concerns. If you are trying to focus on a specific skin issue, serums are used as a direct remedy. Oils are generally used to improve hydration by adding moisture. Both serums and oils can be made from oil combinations of mixtures, but serums go first (as they are lighter) followed by oil (heavier moisturizer).