Orson Pratt Sr. (September 19, 1811 – October 3, 1881) was an American mathematician and religious leader who was an original member of the Quorum of the Twelve Apostles of the Church of Jesus Christ of Latter-day Saints. He became a member of the Quorum of the Twelve of w:The Church of Jesus Christ of Latter-day Saints (LDS Church) and was a leading Mormon theologian and writer until his death.