The New LDS Organist is a free program which has 12 audio based lessons with PDF supplements. This course is designed to help pianists adapt their skills to the organ as soon as possible. It is also available on iTunes as a podcast.
OrganTutor Organ 101 (paid) is a well-designed computer tutorial and a workbook. It has 62 lessons for the pianist on the basics of classical or traditional sacred organ playing.
I hope you will check out these 2 programs for yourself.