perhaps from ὀπτάνομαι (through the idea of the shading or proximity to the organ of vision); the eye-"brow" or forehead, i.e. (figuratively) the brink of a precipice:--brow.
probably from an obsolete (to rise or "rear"; perhaps akin to αἴρω; compare ὄρνις); a mountain (as lifting itself above the plain):--hill, mount(-ain).