Oy is a "billy-bumbler" who joins the ka-tet of Roland Deschain, Jake Chambers, Eddie Dean, and Susannah Holmes. He is named after his attempt to say the word "boy" (Roland's nickname for Jake), which came out elided as "`oy."
Joining the Ka-tet
Oy’s first meeting with the ka-tet is when he wakes up Jake by licking his face. He is skinny and has bite marks on his body, leading Roland to believe he was ejected from his former pack of billy-bumblers, probably for his unusual ability to speak (or imitate human speech). Skittish at first, he is befriended by Jake and follows the ka-tet on their journey. He is especially attached to Jake, and it is hard to tell whether he is Jake’s pet billy-bumbler, or Jake is his pet boy.
Lud and the Calla
Oy helps Roland rescue Jake when he is kidnapped by Gasher in Lud. After this Roland starts seeing Oy as part of their ka-tet, and Roland and Jake especially can't avoid seeing Oy more as a human than an animal.
When the ka-tet reaches Calla Bryn Sturgis, Oy impresses the townspeople by acting as a full part of the ka-tet with the others, calling himself Oy of the Line of Eld.
Jake's Death
In Maine of 1999, Jake is struck and killed by a van in the proces of sacrificing himself to save the life of Stephen King. Oy is with him until the very end, and weeps as Roland buries him in the forest. After this, Oy lies howling on Jake's grave, staying there for tens of minutes. Roland first thinks he will stay by Jake's grave until he starves to death, but he instead decides to follow Roland to the end of his quest. However Oy is never again the same - he is always mourning Jake and barely talks any longer.
In End-World
Oy follows Roland, Susannah Dean and Patrick Danville into the White Lands of Empathica in End-world. As Susannah steps through the door to New York, Oy has the choice to follow her or to stay for the Dark Tower. Oy chooses the Tower and Roland.
A few days later, the trio is attacked by Mordred Deschain as Patrick Danville falls asleep on his post. Oy wakes and attacks Mordred to stop him from getting the sleeping Roland. Roland wakes up in time to see Oy biting Mordred and having the chance to flee, but instead bravely trying again. Mordred throws the bumbler, who is pierced to a broken branch. Roland kills Mordred and goes to the dying animal, who licks his hand and says the one word – `Olan – before dying.
In the Afterword
When Susannah reunites with alternate versions of Jake and Eddie in New York of 1987, she finds that they, now as brothers, own a dog with gold-rimmed eyes whose barks sounds a lot like human speech.