To form oleum from sulfuric acid, you need to react fuming sulfuric acid with sulfur trioxide. The reaction between 100% sulfuric acid and sulfur trioxide gas, SO3, gives rise to oleum, which is a mixture of sulfuric acid and sulfur trioxide. The resulting oleum is typically characterized by a concentration of sulfur trioxide between 20 and 65%, depending on the reaction conditions and desired specifications.