Oy is a billy-bumbler, one of a species of creatures native to Roland's world. 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. While on the road to Lud, Jake rescues a wounded "billy-bumbler". When he first sees it, he calls, "Come here, boy," and the animal mimics Jake's words, barking, "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 helpful in times of need, and sometimes nearly human in both intelligence and compassion. During the quest, especially when a problem arises, Oy is a valuable ally.

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 beyond that of his ability to replicate some words that the others speak. Oy is often referenced as the smartest bumbler that characters have seen since the world had moved on. Oy provides a much-needed shelter from the harsh adultness of the quest for his master, often playing with Jake or providing moments of lightheartedness to the ka-tet. Together, the two are sometimes able to recall the innocence they have lost.

Oy is killed in the last Dark Tower book. He saves Roland's life by attacking Mordred, who came to kill Roland in his sleep. Mordred snaps Oy's back and impales him on a tree limb while the bumbler is defending his friend, but Oy finds enough strength to lick Roland's hand one time before he dies. 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 with "odd, gold-ringed eyes and a bark that eerily resembles human speech."