Old Man Willow was a great willow tree of the Old Forest; he was the root cause of much of the Forest's anger and hatred toward the trespassing of the walking things that pass through there. He may have ever been an Ent who had become tree-like, but was more likely a Huorn, as the Old Forest was originally part of the same primordial forest as Fangorn. He had great power within the Old Forest, and was able to direct all paths to him.
Tom Bombadil had power over Old Man Willow, and checked the evil in him as much as he could, or was willing to.
In The Fellowship of the Ring book, Old Man Willow casts a spell on the hobbits (Frodo, Sam, Merry and Pippin), causing them to feel sleepy. Merry and Pippin lean against his trunk and fall asleep, while Frodo sits on a root to dangle his feet in the water, before also falling asleep. The ill spirit then traps Merry and Pippin in the folds of its trunk. Sam and Frodo attempt to burn Old Man Willow in order to release their friends, but Old Man Willow communicates to Merry, who is still inside his roots, that if the burning does not cease, the hobbits would be squeezed in two. They are saved by the timely arrival of Tom Bombadil, who 'sings' the ancient tree to sleep. Whether or not Old Man Willow is truly evil remains a mystery. It is possible that he represents the anger and bitterness of nature toward those who try to destroy it.