Oy is a billy-bumbler, a primary character in The Dark Tower Series, and a member of Roland Deschain's second ka-tet. Oy first appears in The Dark Tower III: The Waste Lands and appears in every subsequent book.
Quest for the Dark Tower[]
Oy first approached the ka-tet timidly as they camped on the road to Lud, encouraged by Jake Chambers, who quickly befriended him. Jake called to him "Come on, boy!", to which Oy replied with his new name. Oy had been ejected from his pack, possibly due to his aptitude at mimicry, and was happy to follow Jake, who he calls "Ake". Later on in Lud, Oy helped Roland to find Jake after he’d been kidnapped by Gasher. Oy then willingly attempted to sacrifice himself to save Jake at Roland’s direction, marring the Tick Tock man badly in the process, but survived and continued his journey with the ka-tet. During this encounter Oy also demonstrated his intelligence, showing the ability to count how many people were in the room holding Jake.
Oy and Jake develop a special bond that translates into telepathy and the ability to switch bodies as they ran from the old vampires in the bowels of the Dixie Pig. Also in the Dixie Pig he helped Jake hunt down Susannah/Mia’s smell.
Over time his intelligence, already greater than the average bumbler, grew, and he spoke more with more complicated words. At the town hall meeting in Calla Bryn Sturgis, Oy even performed the same self-introduction, including the bow.
Oy comforts Jake in his final moments by licking his face clean of the blood and dirt left when Jake saved Stephen King from certain death. Their final words are not revealed, but it is hinted that Jake passed on Eddie Dean's warning about Dandelo. After Jake’s sacrifice, Oy cried several times, showing that on top of his wits he also had a larger degree of emotional intelligence and was able to form a genuine bond with his best friend. This went far enough that he was willing to abandon Jake’s grave solely at his dying request, even though he didn’t want to.
Although Oy seldom spoke after Jake's death, he said "Olan" and sat at Roland's heel when given the opportunity to escape the treacherous journey through Susannah’s door the night before the remaining group’s arrival at the Tower. His allegiance Jake and their ka-tet and family led to his willingness to sacrifice himself so that Roland would survive the attack by Mordred Deschain.
Roland saw a portent of Oy's death whilst trapped inside Maerlyn's Grapefruit in The Long Road Home and Wizard and Glass, seeing him impaled by a tree branch. Oy still lived, he licked Roland's hand once while he was dying. Oy said "Olan" once, and then passed into the clearing.
In the Epilogue section The Dark Tower, Susannah Dean returns to New York and meets the twinners of Eddie and Jake, named Eddie Toren and Jake Toren. Both already feel like they know Susannah, and Eddie is already in love with her. In the "Coda" section Stephen King states that we can rest assured that a canine version of Oy may enter the picture as well, but this is sadly never confirmed.