Mammoth Uncensored Word List:
34 - 14 letter words starting with op

openhandedness openmindedness operationalise operationalism operationalist operationalize ophidiophobics ophthalmitises ophthalmologic ophthalmometer ophthalmometry ophthalmopathy ophthalmophobe ophthalmoscope ophthalmoscopy opinionatively opisthobranchs opisthocoelian opisthocoelous opisthoglossal opisthographic opisthotonoses oppignorations opposabilities oppositenesses oppositionists oppositionless oppressiveness opsonification opthalmologist optimalisation optimalization optimistically optoelectronic