He is voiced by Junko Minagawa also voiced Sailor Uranus from Sailor Moon Crystal.
Initially, Oz appeared simply as a pair of rabbit dolls completely black in color with tiny beaded red eyes, a small stitched mouth, and a large bow on his chests. Both dolls had round bodies, stubby arms, and disproportionately long arms. They were kept in a seated position, staring ahead of themselves vacantly.
As B-Rabbit, Oz appeared as a massive anthropomorphic rabbit that retained his black fur and red eyes. However, in his new form Oz was clad in a larger version of the outfit Alice provided him when she brought him to the 1st Dimension. This outfit was a red overcoat with a white collar and diamonds formed in a line going down the arm to white cuffs that ended triangularly; all while maintaining the bow Oz had since his creation.
For the majority of the series however, Oz appears identical to Jack when he was fifteen. Though he'd originally only resided within Jack's body because he'd lost his own, Oz's power has since altered Jack's body and made it his own - even appearing within his subconscious as he does in Jack's body rather than his own. As such, Oz is now seen as a 15 year old boy measuring at 164 cm tall, with emerald green eyes (though his eyes do return to their original red form when Oz accesses his power) and golden blonde hair. In this form, Oz often wears a a white dress shirt, a gray vest and black tie, shorts, socks, shoes and overcoat. Additionally, with his powers reinstated, Oz bears the Incuse of an Illegal Contractor over his heart; being a red brand of sorts shaped like a clock that's made at least 6 progressions.
In his early life, Oz had no feelings whatsoever. He merely watched the world around him through the doll that Lacie had brought back to Earth with her, watching Lacie grown from a young child into a young woman. Oz's first emotions seemed to be felt when Lacie returned to the Abyss and announced her death to Oz and the Core of the Abyss, as Oz could feel tightness within him upon hearing her words. Oz fell into slumber until he was awoken by Alice when she bit his ear. Through his time spent with Alice, Oz developed positive feelings despite her aggressive behavior with him, having a sense of happiness spread through him when he saw her smile.
Oz's sense of self-awarence finally formed after Alice gave him his name by abbreviating Oswald's own name (however Levi had to change Os to Oz for her), with this he produced feelings of happiness through the fact that his existence finally meant something. When the Intention of the Abyss approached Oz with a fragment of Lacie to deliver to Jack, Oz felt that he had to return the favor for Lacie, who had given him life. Oz absorbed Lacie's tainted thoughts and feelings of Jack even though it meant one of his bodies would be destroyed. As the Tragedy of Sablier started, Oz wondered if delivering Lacie's thoughts and feelings to Jack was a mistake as he noticed Jack's odd behavior. The two formed a contract, which soon lead Oz to great pain and despair as Jack had him slaughter anyone who got in his way, something which Oz had never wanted. He then got to the point where he was calling out for Alice through his tears, begging her to save him. Oz's own pain worsened after Alice committed suicide to stop Jack from hurting him again, as a great sadness swept over Oz for not being able to protect her as he had promised.
Since adopting the existence of Oz Vessalius, Oz's personality has become more and more human, potentially as a form of mimicry of those he's surrounded himself with. Such as having become bright, optimistic and mischievous through his time as Oz Vessalius. On a darker note, Oz devoted his life to impressing Xai in his early years as Oz Vessalius. Constantly working to gain the praise and recognition from the man whom Oz believed was his father. Showing that Oz has an undeniable need for affection and acceptance. Unfortunately though, because Xai has a vendetta against Oz and Jack because he's aware of Oz's true origins, Oz never received the love he craved and often ended up feeling wounded and helpless. Due to this, he'd always cover up his true feelings under a facade of contentment. As a note, whenever he attempts to charm a girl he's interested in, he tends to go a bit overboard with his compliments and gestures. Such as his first encounter with Sharon Rainsworth.
Although Oz has evolved both mentally and physically, his love for Alice has prolonged until the very end - as he considers nothing to be more important than Alice's safety. Throughout his existence however, he's grown attached to numerous others and devotes his care to them almost equally. The lengths that he's willing to go to in order to ensure the safety of those he cares for is far more than enough to prove his need to protect - likely stemming from his failure to protect Alice during the Tragedy. Even so, just like during the Tragedy, Oz has no desire to kill anyone despite being one of the most powerful forces of the Abyss. Ultimately, having his powers and body stolen and returned to him has helped him in a way, as it has allowed him to fully experience both the tragedy and beauty of the world to its full extent. This may have promped Oz to opt for oblivion rather than change the past in order to maintain the sense of harmony that he believes trumps the perfect world Oswald seems to believe in.
When Oz turns fifteen. During the ceremony to celebrate his legal adulthood, Oz is 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 - a monster born in the Abyss. Oz makes a contract with her so they can both escape the abyss. Upon returning to the real world, he discovers that 10 years have passed. In order to find out what exactly his 'sin' is, he decides to work under Xerxes 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 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. He decided that, 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 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 is able to stop Alice's B-Rabbit's power temporarily on his own; another instance is when Oz is almost kidnapped by the Baskervilles at Lutwidge, and a more recent one is when he was able to summon B-Rabbit's scythe to fight his assailants in 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 him, will most probably interfere with the real world. Later, Oz reunites with Gilbert and immediately meets with the person behind Oz's homicidal nature: Zai Vessalius, his father. it is said that Oz had a desire to destroy everything until Alice stopped him.
- According to Break, Oz may not be the real Oz Vessalius, as during his birth, Oz's father took him away: it is suspected he might have done something to him or exchanged him with another baby. This is proven true when it is revealed that his body was Jack's all along, and that he (Oz) is the true B-rabbit.