Oz is introduced as a romantic interest for Willow Rosenberg. His most outstanding trait is his detached, ironic approach to life, masking a deeply philosophical interior. He is also the lead guitarist for the band Dingoes Ate My Baby, which performs frequently at The Bronze.
In the episode "Phases", Oz finds that he has recently become a werewolf, turning into a monster on the three nights around a full moon, having been bitten on his finger by his young cousin Jordy. On his second night as a wolf, he is nearly killed by a hunter for his pelt, but saved by Buffy Summers and the Scooby Gang, with whose help he quickly finds a way to minimize the danger posed by the wolf: he locks himself into a cage for the appropriate nights, watched over by the Scoobies in shifts.
But the wolf personality gradually contaminates Oz's human side, and although in love with Willow he is irresistibly drawn to another werewolf, Veruca, a fellow student at UC Sunnydale, who unlike Oz considers the wolf to be her true self. After spending a night in the cage with Veruca, and killing her at the following sunset to protect Willow ("Wild at Heart"), Oz resolves to leave Sunnydale so as not to endanger his friends further.
Some months later ("New Moon Rising"), Oz returns to Sunnydale, believing that his lycanthropy is now under control through techniques learned in Tibet. Although the moon no longer affects him, he can still transform into the werewolf when faced with strong emotion, such as anger, fear or pain, as he discovers when he finds that Tara Maclay has replaced him in Willow's affections, and when The Initiative seizes him for its experiments.
After that discovery, Oz leaves Sunnydale for good (reflecting Seth Green's decision to move on and pursue his film career). He is last seen in Willow's dream ("Restless"), as she meets Oz in college. Oz is not seen again for the remainder of the series' run.
He makes an appearance in the follow-up novel, Queen of the Slayers, by Nancy Holder.
The story takes place immediately after the final episode from season 7 - "Chosen". When the gang needs the assistance of the Golden One, they soon find out that their old friend Oz is in Tibet. Oz is 1/10th of a group of Werewolf guardians of the Golden One, the same monk who taught Oz how to control his transformation. When Willow sees Oz for the first time, in a very long time, they catch up on old times and Willow is pleased to hear that Oz has a completely different life than she would have imagined. He's not only learned to control his transformation, but he also appears to effeciently hunt/fight in a controlled pack of werewolves. The werewolf guardians, which never had a formal name/title in the novel, all serve the Golden One and are seen wearing modified helmets, which appear to have muzzles attached.
