Orson Pratt Sr. was an American religious leader and mathematician who was an original member of the Quorum of the Twelve Apostles of the Church of Christ. He became a member of the Quorum of the Twelve of the Church of Jesus Christ of Latter-day Saints and was a leading Mormon theologian and writer until his death.

In the Latter Day Saint movement, the Quorum of the Twelve is one of the governing bodies of the church hierarchy organized by the movement's founder Joseph Smith and patterned after the Apostles of Jesus. Members are called Apostles, with a special calling to be evangelistic ambassadors to the world.