RSO stands for “Rick Simpson Oil,” the namesake of Canadian medical marijuana activist Rick Simpson.
RSO contains lower concentrations of THC than cannabis flower. However, the oil is recommended more for its medicinal purposes than for recreation. Rick Simpson became an advocate for this process after successfully treating his own cancer with the oil. The oil is also unique in that dangerous solvents like butane are not necessary, instead using things like alcohol or even water, then boiling the solvent off using a rice cooker.
Rick Simpson does not sell or patent his process. Instead, he encourages others to create their own oil following his methods, which are available at his official website. Start with the FAQ for directions on making oil.