Christopher Meloni left the show towards the end of the fourth season because he was also working on Law & Order: Unità Speciale (1999) 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.

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