A race of trolls bred by Sauron in the later years of the Third Age, seen among the trees of southern Mirkwood, and the mountain borders of Mordor. They were superior to other trolls in almost every way: stronger and more powerful, but also showing greater intelligence and skill. Many of their abilities they owed to being under the direct will of their master, Sauron, and while under his control they could exist in direct sunlight, unlike others of their kind.

In the battle before the Gates of Mordor, Aragorn and the Captains of the West encountered a company of large trolls, identified as 'hill-trolls out of Gorgoroth'. That battle took place in sunlight, and so it seems to follow that the hill-trolls must have been Olog-hai. They are described as being taller than a man, and covered in horny scales, carrying hammers and bucklers in their claws, though there is no way of knowing whether this description applies to all of the Olog-hai, or just to those in the battle.