This question has quite a complex answer. Facial oils and moisturizers come in all textures, formulas and weights. Facial oils have many benefits to the skin, but the overall purpose is to add an extra layer of protection to your skin barrier. All facial oils and moisturizers are NOT created equal. They can range from extremely lightweight to rich and thick.
First off, let me explain the correct order to use all of your skincare products.
-Micellar water/ Oil Cleanser
-Exfoliant (1-2x weekly either rinse off or leave on exfoliating treatment)
-Mask (sheet masks/hydrating masks can be used daily, deep pore/clay masks 1-2x week max)
-(Thin Oil/Treatment Oil)
-(Thick Oil/Hydrating Oil)
Now where does the oil fit in? Well, it all depends on what type of moisturizer you're using and the thickness of it. All skincare products go in order of thickness, so it should go thinnest to thickest. If you apply a thicker product before a thinner product, the product itself will not be able to absorb into your skin. Think about a puzzle. Each piece has to be supported by another piece in order for them to work together. Same goes for skincare. Each product needs to be layered and supported by the proper product for them all to work together correctly.
Majority of the time you are going to apply your facial oil last in your night time routine and second to last (before your sunscreen) in your morning routine. Our S.O.S oil is a medium weight oil, so if you are using a rich cream the oil will go underneath. If you have a lightweight moisturizer, like a gel or water cream, the oil will go on top! Another rule of thumb- hydrating oils go on top of moisturizers and active treatment oils go underneath moisturizers.
If you are using a treatment oil (one that contains salicylic acid, retinol, active ingredients, etc.) they will always go under your moisturizer. It needs to be able to penetrate the skin and work directly into the epidermis. If applied on top of your moisturizer, it won't be able to treat your skin properly. The molecular structure of acids are smaller than rich creams, so they need to go underneath.