O.P.A.P. is, very simply, the acronym for “Onomasia Proelefsis Anoteras Poiotitas” that you could translate in English by “Appellation of Origin of Higher Quality”.
Greek wine law is kind of based on the French legislation regarding wines. Legally, it means that the designation O.P.A.P. that you could find on the Label of Greek wines would be an equivalent to the French quality wine category known as VQPRD (in French: ”Vin de Qualité Produit dans une Région Déterminée”).
Greece counts 20 O.P.A.P. Appellations only, because of stricter conditions applied to this designation, meaning that wine with this distinctive designation will most likely be of a higher hand, and will show off more of a "Terroir" taste.