Mammoth Uncensored Word List:
67 - 18 letter words starting with o

obstreperousnesses octingentillionths octogintilliardths octotrigintillions octovigintillionth oculocraniosomatic oligodendrogliomas operationalisation operationalization ophthalmologically opinionativenesses organophosphoruses orthogonalizations orthopsychiatrists orthosymmetrically oscillographically ostentatiousnesses otherworldlinesses overapprehensively overcapitalisation overcapitalization overcentralisation overcentralization overclassification overcommercialized overcommercializes overcommunications overconcentrations overdecorativeness overembellishments overextrapolations overfertilizations overgeneralisation overgeneralization overidentification overillustratively overimpressionable overimpressionably overindebtednesses overindustrialises overindustrialized overindustrializes overintellectually overinterestedness overinterpretation overlasciviousness overlicentiousness overneglectfulness overneutralization overoptimistically overpresumptuously overprocrastinated overprocrastinates overproportionated overprosperousness overprotectiveness overrepresentation overrepresentative overscrupulousness oversentimentality oversimplification overspecialisation overspecialization oversusceptibility oversuspiciousness overtheatricalness ovolactovegetarian