Ossa | Ancient, Pelion, Mythology | Britannica Ossa, mountain massif, nomós (department) of Lárissa (Modern Greek: Lárisa), eastern Thessaly (Thessalía), Greece It lies on the Gulf of Thérmai (Thermaïkós) and is separated on the north from the Olympus (Ólympos) massif by the Vale of Tempe (Témbi)