[The Saucepan Man]

... Oliphaunt.

Yes, O is for Oliphaunt, an enormous elephantine creature native to Southern Middle Earth, used by the Haradrim as war-machines (Mumakil).

To his great delight, Sam caught sight of an Oliphaunt in South Ithilien accompanying a column of Haradrim marching towards Mordor, confirming the existence of a creature of which he had only previous heard of in a Hobbit nursery-rhyme.

These creatures were subsequently employed by the forces of Sauron at the Seige of Minas Tirith and the Battle of Pelennor Fields.