Mammoth Uncensored Word List:
14 - 20 letter words starting with o

octogintacentillions octotrigintillionths olivopontocerebellar onychoosteodysplasia otorhinolaryngologic overapprehensiveness overdifferentiations overenthusiastically overintellectualized overintellectualizes overpresumptuousness overprocrastinations overprotectivenesses overrepresentatively