Mammoth Uncensored Word List:
30 - 15 letter words starting with op

openheartedness openmouthedness operationalised operationalises operationalisms operationalists operationalized operationalizes operativenesses ophthalmography ophthalmologies ophthalmologist ophthalmometers ophthalmophobes ophthalmophobia ophthalmophobic ophthalmoplegia ophthalmoscopes ophthalmoscopic opinionatedness opisthognathism opisthognathous opisthographies opportunenesses opprobriousness opsonifications opthalmologists optimalisations optimalizations optoelectronics