Orson Pratt Sr. (September 19, 1811 – October 3, 1881) was an American religious leader and mathematician who was an original member of the Quorum of the Twelve Apostles of the Church of Christ (Latter Day Saints).

He was the younger brother of Parley P. Pratt, who introduced him to the LDS Church and baptized him on Orson's nineteenth birthday, September 19, 1830, in Canaan, New York.

[2] Pratt was ordained an Elder several months later, on December 1, 1830, by Joseph Smith and immediately set out for Colesville, New York, his first mission.

[4] On February 2, 1832, he was ordained a High Priest by Sidney Rigdon,[5] then he continued his missions, preaching in Pennsylvania, New York, New Jersey, Vermont, New Hampshire, Connecticut, and Massachusetts.

He contributed to the mission by preaching in Scotland, and producing an early missionary tract, "An Interesting Account of Several Remarkable Visions and the Late Discovery of Ancient American Records.

"[8] This tract contains the earliest known public printing of an account of Smith's First Vision and also contains material similar to that later published as the 1842 Articles of Faith.

Rumors and gossip were rife in Nauvoo, Illinois, and Pratt found the religious principle of plural marriage difficult to accept.

"[10] To counteract these allegations, Joseph Smith compiled a pamphlet of affidavits, certificates, and letters which proved his innocence and Bennett's guilt concerning sexual misconduct.

[13] After the death of Joseph Smith, Pratt was among the apostles that supported the leadership of Brigham Young, who determined to move his followers from Nauvoo to the Salt Lake Valley, where the LDS Church became established.

Young also opposed Pratt's teachings that the One God of the Bible was a matrix of attributes shared by many individuals who had achieved exaltation to godhood.

[21] After settling in the Salt Lake Valley with the Mormon pioneers, Pratt was called to return to Europe as a mission administrator between 1848 and 1851, during which time he also served as editor of the Millennial Star.

"[23] Although this goal was not achieved, by the end of the year there were twice as many LDS Church members residing in Britain as in the United States.

[citation needed] As a mission president, Pratt was admired by the missionaries for the many logical and well-written pamphlets he wrote and published while he was there and the way he communicated with British intellectuals to discuss theology.

[26] When news reached Pratt that the LDS Church was moving westward, he said, "We do not want one saint to be left in the United States."

He made regular readings with the company's scientific instruments, took notes on geological formations and mineral resources, and recorded information on plants and animals.

"[29] As a mathematician, Pratt assisted company scribe William Clayton[30] in the design and invention of a version of the modern odometer.

[32] With Erastus Snow, Pratt entered the Salt Lake Valley on July 21, 1847, three days ahead of the main body of the Vanguard company.

[37] On August 29, 1852, Orson Pratt delivered a sermon that taught the principle of polygamy, discussing the blessings of raising many children up in the LDS Church.

[47] In the local and Mormon press, Sarah Pratt was accused of having had an adulterous relationship, not with Smith, but with John C. Bennett, and several affidavits were printed in local and pro-Mormon Nauvoo publications,[48][49] including the leading councils of the church and from others such as Jacob B. Backenstos, a relative of the sheriff of Hancock County, though many of these allegations are believed to be falsified.

[4] At the time, apostle Wilford Woodruff stated that "Dr. John Cook Bennett was the ruin of Orson Pratt.

Pratt wrote a postscript to his brother's letter: "J.C. Bennett has published lies concerning myself & family & the people with which I am connected".

[59] In Utah, Pratt's strong skills in analysis and writing led Young to assign him to produce sermons and pamphlets dealing with religious topics.

In "Absurdities of Immaterialism", Pratt defended the Mormon doctrine of materiality, with reference to science, philosophy, and theology.

At a special conference in Salt Lake City in August 1852, Pratt publicly preached a sermon announcing the doctrine of plural marriage.

In 1865, a majority of the First Presidency and the Quorum of the Twelve Apostles of the LDS Church officially condemned some of Pratt's doctrinal writings, including some of his articles from The Seer.