fo stands for fragrance oil.
a fragrance oil can be made up of a mixture of essential oils, synthetic scents, or even could be all one or the other.
i'm having a "thing" with my sister, who i plan to make soap with. she is into earthy, and i'm into what smells good. she is stuck on essential oils, and is being kind of snobby to my fragrance oils. i have read that some essential oils are technically worse than some fragrance oils, due to the extraction techniques/compounds used to extract the oils. not that i believe either is bad for you, really, but relatively speaking.....