There are many differences between serum and face oil and both are essential for healthy skin. Serum has light weight, high skin penetration, targets a specific skin concern and has high % of active ingredient. Serum is used after cleansing and toning the face. Whereas, face oil is heavy weight and seals the moisture within the skin. The oils are important too because their composition is similar to sebum and helps protect the skin barrier. Since oil is heavy and have large molecular weight, it is applied last in your skincare routine and preferably at night.
Chose a serum based on your skin concern and oil based on your skin type. Some of the facial oils like Rose hip oil, Hemp oil, Jojoba oil are all-rounders and can be used for all skin types.