Mrs. O'Leary was born from Cerberus and Nyx. She was originally Quintus' hellhound pet. He tamed her and they became very close. According to Quintus, the story of how he got Mrs. O'Leary involves "many close calls with death and quite a few giant squeaky toys."Camp Half-Blood with her owner Quintus. When Percy first sees her, he attacks her but is stopped by Quintus who wouldn't let anything happen to his pet. Mrs. O'Leary turns out not to be violent, but a very sweet and friendly, making friends with Percy and Tyson, and often playing with a shield as a Frisbee and a combat dummy to play fetch. When Percy is about to journey into the labyrinth, Quintus gives him a whistle made of Stygian Ice so if Percy ever needs help, he could use it to call Mrs. O'Leary.
In Antaeus' Arena, Percy used the whistle to call Mrs. O'Leary for help to save himself, Annabeth Chase, Rachel Elizabeth Dare and Ethan Nakamura, but she disappeared after they fled from Luke Castellan's minions. Mrs. O'Leary found her way back to Quintus, who was revealed to be Daedalus who had originally built the labyrinth. When they were attacked by Luke's monsters, Percy, and his friends fled, but Quintus wouldn't leave his pet. They made it out and joined the Battle of the Labyrinth at camp with Mrs. O'Leary saving Percy and Annabeth from Kampê. With the help of Nico di Angelo, son of Hades, Daedalus passed on and left Mrs. O'Leary in Percy's care.
When Percy left camp, he left Mrs. O'Leary in the care of Charles Beckendorf who forged her a collar, a toy, and other useful items.
Mrs. O'Leary arrives in the middle of Percy's English exams and the Mist makes everyone believe she was a poodle, which impressed Percy due to her size. He follows her into the park where he comes across Thalia Grace and Nico who, along with Percy, were all taken to the Underworld. Mrs. O'Leary tries to warn him, but she was too late.
Percy is ambushed by a gleeful Mrs. O'Leary who is happy to see him at camp. He becomes sad as he remembers how Beckendorf (who died a few hours ago on the Princess Andromeda) would take care of her when Percy would be away for a quest or vacations. Beckendorf had also welded Mrs. O'Leary's favorite bronze chewing bone and her collar. Pushing the sad thought away, Percy follows her until he found Nico with Juniper and Leneus. Nico takes the hellhound for a walk while the others talk because Juniper and Leneus were both afraid of dogs. Nico gets Percy to leave with him afterward to Luke's mother's house. Percy rides on Mrs. O'Leary who uses shadow travel which tires her. After the meeting, they met Hestia who gives Mrs. O'Leary a large dog treat. Due to shadow traveling, the huge hellhound yawns and takes a nap.
With the help of Grover, the group is able to get to the Underworld where Mrs. O'Leary happily played with Cerberus. Percy calls her off when the Furies came to collect them. After Percy and Nico escape, she transports them to the River Styx where Percy baths to receive the Curse of Achilles.
Mrs. O'Leary takes part in the Battle of Manhattan, but got injured fighting the Drakon. She gets a message from Percy to Nico for reinforcements and takes an injured Chiron back to camp. Later, Percy had to face a hellhound but hesitated to kill it. He reminded himself: "This is not Mrs. O'Leary. This is an untamed monster. It will kill me and all my friends."
In one of his dreams, Percy sees Tyson riding her as they look for him. Percy then whistles for her when he and his friends are right near Camp Jupiter and she helps them haul tons of Imperial Gold and fights in the battle, and attacks a Gorgon. Once the battle is over, Percy watches her play with Hannibal and says how she has finally found a playmate who she can rough-house with.
Mrs. O' Leary is first seen among the mob in New Rome, carrying Tyson into the city. She later enters the Forum, while Ella rests on her back. After Ella's sudden outburst, Percy tells Tyson to take Mrs. O' Leary and Ella someplace else and they leave by shadow traveling away. Percy later informs his companions that he told Tyson to take Ella to Camp Half-Blood. Chiron then informs the group that Ella, Tyson, and Mrs. O' Leary had arrived safely.
Mrs. O'Leary was mentioned by Percy while in Tartarus. He considered calling her for backup, during his and Annabeth's fight with the empousai, but decided against it, as he was worried that Tartarus could have a negative effect on her and revert her personality to a fearsome monster. As a result, she did not ultimately appear.
When Percy arrives at Camp Half-Blood for the weekend, he uses Mrs. O'Leary to shadow travel to the top of the Colossus's head in the middle of the battle. Mrs. O'Leary then freaks out, as she isn't "fond of heights" and pees on the Colossus. She, Percy, and Chiron then start distracting it and luring it into the ocean. When the giant statue is defeated by a plague arrow from Apollo, the Colossus falls towards the group. While Percy and Chiron start getting out of the way, Mrs. O'Leary shadow travels away to avoid getting crushed.
Mrs. O'Leary is sweet, energetic and playful. She loves eating, running around, and playing with chew toys. Mrs. O'Leary adores her owners, Quintus and Percy. She is always ready to help them and acts protective, as shown when Hades and his skeletons attempt to attack Percy after his bath in the Styx.
Like all hellhounds, Mrs. O'Leary is afraid of heights. When frightened, she "wet" herself on top of a foe's head.
Mrs. O'Leary is very scary and described as a mastiff. She is unnaturally large, even for a hellhound. Percy describes her as "bigger than a tank and barks slightly louder than an artillery gun". Mortals, on at least one occasion, see her as a large standard poodle, rather than an impossibly big dog.
- Shadow Travel: Like all hellhounds, Mrs. O'Leary can travel through shadows. However, it takes a lot of energy, especially the first couple of times she uses it, and she usually must rest afterwards.
- Enhanced Physical Prowess: Like all hellhounds, Mrs. O'Leary has superhuman strength and speed.
The name Mrs. O'Leary likely comes from a woman from Chicago, who was rumoured to have started the Great Fire of Chicago.
- Whether her personality and friendliness would be affected by her rebirth is unknown.
- When Percy was questioned about having a hellhound for a pet, he revealed it was a long story just like Daedalus told him, involving a lot of close calls with death and a few giant chew toys.