Oz Vessalius is a 15-year-old boy with blond hair and green eyes and is from the prestigious Vessalius family, whose father, Zai, is the reigning lord. His mother was killed by the Nightray family, but despite this, he retains a bright, optimistic, and energetic personality. The story of Pandora Hearts starts when Oz turns fifteen. During the ceremony to celebrate this reaching of adulthood, he was dragged into the Abyss for the 'sin of his existence'. There he meets a young girl named Alice who turns out to be a Chain. Oz makes a contract with her in order to escape the Abyss. Upon returning to the real world, he finds out that 10 years have passed. In order to find out what his sin meant, he began to work under Break as part of Pandora. During this, it is shown that when he was young, he worked hard to gain praise and recognition from his father. However, he had discovered that his father hated him to the point of not wanting to touch him. After experiencing this, Oz began to think that there is no guaranteed thing in the world. He thought of himself as weak and, in order to protect people, he should at least remain an obedient child and let harm come his way. After traveling into the Cheshire Cat's lair, Oz meets Jack Vessalius and in turn becomes his 'medium'. Jack eventually speaks at Pandora's headquarters and the public hail Oz as a 'hero'. Oz finds that he can swap control of his body with Jack if he calls hard enough and may sometimes even see snippets of Jack's memories. According to himself he is quite skilled with the sword, particularly in self defense. However, he has very little real experience in fighting. Oz likes cute, younger girls and flirts with them frequently. He once wished to marry Sharon when they first met. Oz, himself, thinks of Alice as his precious someone, calling her his "sun". He believes that Alice is the reason that he can still smile.
- Jack tells Oz that he has the "right" or "prerequisite" over Alice's powers and that a seal won't restrain Alice's powers any longer, giving Oz the ability to use B-Rabbit's power, even without Alice's permission or Gil's help. Oz proves it when he was able to stop Alice's B-Rabbit's power temporarily on his own; another time when Oz was almost kidnapped by the Baskervilles at Latowidge, and then recently when he was able to summon B-Rabbit's scythe to fluently fight his assailants inside Sablier's illusion, despite the fact he had never used a scythe before. According to Lotti, Oz was supposed to be the vessel for the sake of hiding the existence of Jack Vessalius and should the vessel be broken, Jack’s existence will be brought to light and the further warping of abyss’s will, who seeks for him, will most probably interfere with the real world. Later, Oz reunites with Gilbert and immediately meets with the person behind Oz's suicidal nature: Zai Vessalius, his father. It has been seen in chapter 45 that Oz has had a will to destroy everything until Alice stops him. In chapter 46, however, according to Break, Oz may not be the real Oz Vessalius, as during his birth, Oz's father suddenly took him away and perhaps did something to him or changed him for another baby.
- Oz's Seiyū is Junko Minagawa.