Etymology: < ancient Greek ὀρο-, combining form (in e.g. ὀροβάγχη Orobanche n.) of ὄρος mountain, high ground < ὀρέσθαι to raise oneself < the same Indo-European base as Sanskrit ṛṣva high, sublime, Avestan ərəšuua high, lofty. In form oreo- after Hellenistic Greek ὀρεο-, combining form (in e.g. ὀρεοσέλινον mountain parsley) < ancient Greek ὄρειος of or from the mountains < ὄρε-, ὄρος mountain + -ιος, suffix forming adjectives.Combined with second elements of Greek origin. Formations in English are found from the late 18th cent., earliest in orometric adj. Compare post-classical Latin oro- (formations in which are found from at least the late 17th cent.), oreo- (formations in which are found from at least the end of the 18th cent.), French oro- (formations in which are found from the late 18th cent.), oréo- (formations in which are found from at least the second half of the 19th cent.), German oro- (formations in which are found from the late 18th cent.), †oreo- (formations in which are found from at least the mid 19th cent.).

Etymology: Irregularly < classical Latin ōr-, ōs mouth (see oral adj.) + -o- connective.Attested in a small number of scientific (chiefly medical) formations in English from the late 19th cent. Combined with second elements of Greek or Latinorigin.