Mammoth Uncensored Word List:
19 - 12 letter words starting with om

ombrophilous ombrophobics ombrophobous ombudsperson omissiveness ommatophores ommetaphobes ommetaphobia ommetaphobic omnificences omniparities omnipotences omnipotently omnipresence omnisciences omnisciently omnivorously omphalomancy omphalophobe