Of the Mountain
OREIOS (Oreus) was a mountain-god of central Greece--probably Mount Othrys or Oita in Malis or else the Pindus mountain range. He was the father of Oxylos "Mountain-Forests" and Hamadryas "Joined-with-Tree" who, in turn, were the parents of a bevy of tree-specific Hamadryad Nymphs.
Presumably a son of GAIA like the other mountains
CLASSICAL LITERATURE QUOTES
Athenaeus, Deipnosophistae 1. 78b (trans. Gullick) (Greek rhetorician C2nd to C3rd A.D.) :
"The Epic poet Pherenikos (Pherenicus), a Herakleto by birth, declares that the fig (Sykon) was named from Syke (Fig-Tree), the daughter of Oxylos (Beech-Tree); for Oxylos, son of Oreios (Mountain), married his sister Hamadryas (With-Oak) and begot among others, Karya (Walnut-Tree), Balanos (Oak-Nut Tree), Kraneia (Cornel-Tree), Morea (Mulberry-Bush), Aigeiros (Poplar-Tree), Ptelea (Elm-Tree), Ampelos (Grape-Vine), and Syke (Fig-Tree); and these are called Nymphai Hamadryades (Hamadryad Nymphs), and from them many trees derive their names."
- Athenaeus, Deipnosophistae - Greek Rhetoric C3rd A.D.