In Cennini's treatise (Merrifield translation) olive oil is mentioned only to be "used for greasing things not required to dry". According to Gettins and Stout (Painting Materials: A Short Encyclopedia) resins dissolved in olive oil are only known to have been used medicinally, never as varnish.
It's unnecessary, however, to delve too deeply into historical sources in order to argue the unsuitability of olive oil as a paint vehicle because it lacks the fundamental chemistry- unsaturation in the form of double-bonds- that makes a "drying oil".
Please ask the poster who claims extensive use of olive oil as a paint vehicle to cite a verifiable, authoritative source. There's not much point in further discussion otherwise.
Utrecht Art Supplies
Art Materials Information Specialist