Mammoth Uncensored Word List:
17 - 15 letter words starting with os

oscillographies osmoregulations osmotrophically osphresiophobes osphresiophobia osphresiophobic ostensibilities osteoarthritics osteochondritis osteochondrosis osteomyelitides osteomyelitises osteopathically ostraconophobes ostraconophobia ostraconophobic ostreiculturist