Orson is introduced in the final episodes of the second season of the series, and becomes the main mystery of the third season. MacLachlan left the main cast in season six, but made guest appearances until the eighth and final season.
Orson's introduction happens during the final episodes of season two, and he becomes a central figure in the third season's mysteries. MacLachlan departed from the main cast after season six but continued to make appearances in later seasons.