Mammoth Uncensored Word List:
26 - 15 letter words starting with or

oraculousnesses orchesographies orchestrational orchiorrhaphies organochlorines organographical organographists organomagnesium organomercurial organometallics organophosphate organotherapies orientationally ornithodelphian ornithodelphous ornithorhynchus orthochromatism orthodontically orthogonalising orthogonalities orthogonalizing orthophosphates orthophosphoric orthopsychiatry orthoscopically orthosymmetries