A facial oil is very natural ie the Jojoba Company one is all Jojoba Oil, and the numerous Rosehip Oil options you can buy are, for the most part, pure Rosehip Oil – some brands combine oils too.
A serum is very different. Think of it as a moisturiser on steroids – they’re designed to work with specific concerns in some cases ie some will fade pigmentation, others are extra hydrating to help dry skin and wrinkles and so on. They’re super-concentrated with active ingredients, hence they’re more expensive, per mls, than a moisturiser will be.
Personally I use a Rosehip Oil in the morning, and a good serum at night. Both before moisturiser. Some BH members use more than one serum, as they want to treat more than one skin concern, others use oils and serums together too.
Have a good look on here too, there’s heaps of articles BH have posted about facial oils and serums, and what they are designed to do, plus recommended ones to use