Oreius was the brother of Agrius with whom together were known as the "Bear Twins."
They were the illegitimate spawn of Polyphotne whom Aphrodite commanded must marry but chose not to, instead fleeing to Artemis. Artemis allowed Polyphonte to join the ranks of her Hunters of Artemis, but Aphrodite put her in a trance and made her fall in love with a bear. After falling in love with the bear, Artemis, in disgust, threw her out of the ranks of her Hunters by turning the animals against her. She fled back to her father where she gave birth to Agrius and Oreius. They were cannibals and disrespected humans and gods alike.
Zeus ordered Hermes to punish the two in whatever way the latter pleased. Hermes at first decided to dismember them, but Ares did not want this to happen to his grandsons. Instead, Hermes and Ares turned the twins into carnivorous birds where Agrius was turned into a vulture and Orius was turned into an eagle owl.
Oreius and his twin traveled with Luke Castellan as his 'assistants' aboard the Princess Andromeda, and they first came into contact with Percy Jackson and his friends while they were trying to rescue Grover Underwood from Polyphemus. Luke told them their stories and though it was said they like half-bloods for food it is unknown as to whether any unfortunate demigod has suffered such a fate.
After the completion of their mission, both Percy and his friends encountered Oreius and Agrius once more aboard the Princess Andromeda. While Luke crossed blades with Percy, Agrius was to hold Blackjack and Oreius was commanded to eat Annabeth and Grover. Though he relished the thought he never received the chance as the timely arrival of Chiron and the Party Ponies (a group of centaurs) quickly ended his life with an arrow through the mouth sending him to Tartarus only seconds before his brother followed.
The Bear Twins were described as being eight feet tall, wearing nothing but blue jeans, being cover by shag-carpeted thick brown fur, with claws for fingers, a snout for a nose, and all their teeth being pointed canines. Oreius was depicted as being very slow and unintelligent (or at least not as dominant as Agrius) as his typical (if not only) dialogue is "he-he." Agrius often tells Oreius to punish himself which entails Oreius pounding his head against a wall or causing other bodily harm. He is also the only Bear Twin to get hurt but not killed, such as Tyson punching him into the pool.
- It is possible that Chiron himself defeated Oreius, but cannot be confirmed as the book never clarifies just exactly who shot the fatal arrow.
- While being the dominant monster characters of the The Sea of Monsters, the Bear Twins never fight Percy or his friends, though they do hold them captive.
- Oreius and Agrius did not appear in Percy Jackson: Sea of Monsters and were replaced by a Manticore.