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Question: Was Oliver Cowdery aware that some in the Church were practicing polygamy in 1835 at the time he authored the "Article on Marriage"?
On July 7, 1878, Joseph F. Smith discussed Oliver's awareness of polygamy at the time of this publication:
To put this matter more correctly before you, I here declare that the principle of plural marriage was not first revealed on the 12th day of July, 1843. It was written for the first time on that date, but it had been revealed to the Prophet many years before that, perhaps as early as 1832. About this time, or subsequently, Joseph, the Prophet, intrusted this fact to Oliver Cowdery; he abused the confidence imposed in him, and brought reproach upon himself, and thereby upon the church by "running before he was sent," and "taking liberties without license," so to speak, hence the publication, by O. Cowdery, about this time, of an article on marriage, which was carefully worded, and afterwards found its way into the Doctrine and Covenants without authority. This article explains itself to those who understand the facts, and is an indisputable evidence of the early existence of the knowledge of the principle of patriarchal marriage by the Prophet Joseph, and also by Oliver Cowdery. 
However, there continues to be debate about whether Oliver Cowdery knew about--or prematurely practiced--plural marriage in the 1830s.  Oliver would learn about the Fanny Alger marriage, but his reaction at the time seems to have been wholly negative.
The original D&C 101 article outlined the general practice of performing a Latter-day Saint wedding, explained LDS beliefs about the marriage relationship, and denied that the Saints were practicing polygamy.
- Joseph F. Smith, Journal of Discourses 20:29.
- Hales, Joseph Smith's Polygamy Vol. 1, 156–158.