Old Man Willow:

An old willow tree standing by the banks of the Withywindle in the Old Forest. He is a bit of an enigma. Old Man Willow is most probably one of the Black Huorns of the sort mentioned by Treebeard. He seemed to obey Tom Bombadil.

Onondrim:

The elvish name for the race of Ents.

Olog-hai:

A race of trolls. The Olog-hai were different from most trolls in that they were cunning and able to stand the sunlight. Most likely they spoke the Black Speech. They were bred by Sauron and it was unlikely that they survived for long after his downfall.

The Lord of the Rings
The Silmarillion
The Unfinished Tales
The Hobbit
The Complete Guide to Middle Earth by Robert Foster