On the road to Lud, Jake finds a wounded "billy-bumbler". These animals are described as looking like a cross between a raccoon and a dog, with a corkscrew-spiral tail. Introduced in The Waste Lands, the third book in the series, Oy befriends one of the main characters, Jake Chambers. When Jake first sees the bumbler, he calls "Come here, boy," and the animal mimics the sound with "Oy". The bumbler is friendly and intelligent -- Roland explains that it used to be common for billy-bumblers to speak and even be able to perform simple math -- and Jake decides to call him Oy, after the first word they heard him say. Seemingly a mere pet at first, Oy proves to be strangely helpful in times of need and nearly human at times in both intelligence, and compassion.

At the time of their meeting, Roland concedes that Oy may quite possibly be meant as another member of their ka-tet. Later, in the 6th book, this is confirmed when the Tet Corporation forms as the association of "Deschain, Dean, Dean, Chambers, and Oy."

Oy develops emotions and even a sense of humanity past that of his ability to replicate some words that the others speak. Oy is often referenced as the smartest bumbler that people have seen since the world had moved on. Oy, much like Jake, shows times when he's able to retain his original instinct and innocence that he had before the quest while playing with Jake or during times of rest for the gunslingers. During the quest or when a problem comes up Oy is a valuable ally.

Oy finally dies in the last Dark Tower book. He saves Roland's life by attacking Mordred who came to kill Roland in his sleep. He is torn in half and impaled on a tree limb however, Stephen King hints that Oy will be found in the same universe that Susannah travels to and will be in some form of a dog.