OPHIOPHAGUS is a 11 letter word that starts with O.
\O`phi*oph"a*gus\, n. [NL. See {Ophiophagous}.] (Zo["o]l.) A genus of venomous East Indian snakes, which feed on other snakes. {Ophiophagus elaps} is said to be the largest and most deadly of poisonous snakes.
WordNet Dictionary