R.S.O. stands for Rick Simpson Oil — made famous by Rick Simpson, who claimed to cure his skin cancer using a home made concentrated oil and applying regularly to the carcinoma. There is controversy over the solvent he used — naphtha oil — because of its high flammability and association with oil and coal (source: Brave New Weed). The acronym is widely used although not exclusive in its definition.
Essentially RSO is just a whole-plant concentrate that extracts a full profile of cannabinoids and terpenes that may be holistically beneficial due to the “Entourage Effect” — a scientifically debatable but anecdotally strong argument for a sum of parts affect from the whole plant. Other whole-plant oils have successfully been created by using cleaner solvents like coconut oil and hypercritical CO2.
The acronym RSO is sometimes used by brands, because of the widespread awareness of the name, despite not being associated with Rick Simpson or naphtha solvents.