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On Buffy the Vampire Slayer Oz is a werewolf as discovered in the episode Phases. Oz
got bit by Jordie his little cousin who had just grown teeth. It is not known for sure how Jordie gained lycanthropy but his parents seemed to give up the information quickly and easily when Oz asked. To me this suggests that it could of happened quite a while ago and the family are quite accustomed to it now, but as Jordie is only just getting teeth the likelness of a small child surviving a werewolf attack is slim. The other option is that one of the parents have been a werewolf for quite a while and this was passed onto their son, but of course there is no way to know is this could happen. Never the less the lycanthropy was passed onto Oz. He finds this out when he wakes up naked in the woods on the morning of the full moon. I have always wondered what happened on the night before when he attacked Cordelia's car, did the wolf go back to his house after the night. Also Oz's transformation on the last night seemed quite distressing yet he did not notice it on the first two nights? However, from then on Oz would change into wolf form on the night of the full moon and the two days surrounding.
According to BtVS the only way to kill Oz in wolf form is with a silver bullet as in Phases Cain the werewolf hunter tries to do. He of course is stopped and the wolf is subdued easily with a large tranquiliser dart said to be able to take a small elephant down - however on a couple of occasion he has required more than one shot which has not seemed to have effected him. Very large doses of tranquiliser can kill. He has also been captured by the Initiative who systematically tortured him with what appeared to be electric charges. He also survived this but was heavily weakened when he finally escaped.
The Initiative torture happened after Oz had travelled the world. Through a number of different meditations, magics, herbal remedies and incantations he managed to surpress his wolf form on the three nights of the full moon. Unfortunatly this brought forward a new side effect. It moments of hightened emotion Oz would change into the wolf. This is why the Intiative tortured him, to explore the treatments that would turn him into werewolf quite possibly so they could work out how to surpress the beast themselves like they did with Spike. Unfortunatly the methods they used would probably of eventually killed Oz.
Oz so far has managed to never kill a human being. He has thought to have done twice where in fact the first time was Angelus and the second was his friend Pete who had turned himself into a monster. He has however eaten the zombie Jack O'Tool in The Zeppo without his knowledge and killed the other werewolf Veruca when she tried to attack Willow in Wild at Heart. We do not however know how he was keeping himself locked up in his months away from Sunnydale and do not know if harmed anyone in that time. It could be likely that he is not even aware of it, though whenever he got out in Sunnydale he would thoroughly check the papers for casulties.
For actual details of real life lycanthropy and werewolfism visit these links:
Werewolves ~ Lycanthropy
The Shapeshifter and Werewolf Handbook