An ogre emperor, O, is a monster that appears in dNetHack and notdNetHack. He is a strong ogre that is also orcish; an ogre emperor can also be seen via infravision, will seek out and pick up gold, gems and other items, and is an overlord to his kind.
An ogre emperor has a strong weapon attack.
Ogre emperors are not randomly generated, and are not a valid form for normal polymorph.
Only one ogre emperor normally appears in the Mordor Ruins variant of the Chaos Quest. The ogre emperor resides in a throne room within the eastern section of the Mordor Fortress, sitting on the throne himself. He carries The Mantle of Wrath, an artifact orcish cloak, and is also generated with a tsurugi and a war hat. As an overlord to his kind, the tsurugi will always be at least +1, and the armor will be at least +0.
A character strong enough to fight their way to the ogre emperor should be more than capable of defeating him after clearing out the assorted orcs and other monsters in his court - the most that some characters might need to look out for is his MR score of 90 if they are playing a magic-reliant build.