“During the monumental and exact year that began on April 7, 1829, when Joseph Smith began dictating the inspired text of the Book of Mormon, and ended on April 6, 1830, when the Church of Christ was organized, no one was more involved in the key events of the restoration than Oliver Cowdery. He was present for the translation of the Book of Mormon accomplished in an amazing ten weeks through the gift and power of God; he was with Joseph when John the Baptist and later Peter, James, and John appeared as resurrected personages and restored the priesthood through the literal laying on of hands (with Oliver becoming the first person baptized in this dispensation); and he was one of three witnesses called to see the angel and the plates and to testify of the truth of the book. Not only that, but he also prepared the printer’s manuscript of the Book of Mormon and assisted with the printing at a time when Joseph was generally not present. Clearly, it is no exaggeration to call Oliver the cofounder of Mormonism.”
http://maxwellinstitute.byu.edu/publications/jbms/?vol=16&num=1&id=428#2