A general rule of thumb is start with the lightest product you have and end with the heaviest, always leaving your SPF for last. Therefore apply any toners or water-based products first (such as hyaluronic acid) and then use the facial oil. If you have a water-based moisturiser you may find the facial oil is not required. However, if you want to keep using a moisturiser and the oil, then apply the moisturiser and then the oil.