 Christopher Meloni (qv) left the show towards the end of the  fourth season because he was also working on  _"Law & Order: Special Victims Unit" (1999)_ (qv) and he felt that  he could no longer do both shows at the same time. On the show, it  was explained that Keller was transferred to another prison. But  Meloni missed "Oz" and decided to return. He decided that being able  to work on two great shows simultaneously was worth the physical and  emotional toll.