Oscar Wilkes Collet was born August 4, 1821 in Mona Farm, Illinois, near Alton. His parents, Robert and Sophia Catherine Hewitt Collet, were both born in England and immigrated to America, changing the family name to Collet from Corlet. After the birth of Collet's siblings in Mona Farm, two brothers, Irwin Austin and Robert Edgar, and a sister, Emma Sophia, the Collet family moved from Illinois to St. Louis, Missouri.
In 1833, Collet began his schooling at Saint Louis University at age twelve and continued until age eighteen. The records do not show that he received a degree. After his time at school, he took a trip to Europe during the years of 1841 and 1842.
Collet married Agnes R. Dunlap, who immigrated to America in 1847 from Ireland. Agnes was born in November of 1827 to a Scottish father and an Irish mother. Oscar and Agnes were married in the St. Louis Cathedral by Archbishop Peter Richard Kenrick on November 2, 1850.
Collet's first child, Oscar Aloysius, was born in 1851 in New York. Collet's second child, Thomas Vincent, was born in St. Louis in May, 1854. Between 1855 and 1862, four more children were born: Emma, Jeannette, George A., and Agnes. Madelaine, the youngest child, was born in 1863. Most of the children died young and all seven children preceeded their parents in death.
Like their father, Collet's two oldest boys also attended Saint Louis University, beginning in 1864. Thomas only went for four years until 1868. Oscar A. continued for six years until 1870. Oscar A. planned to graduate with honors from the University, but he had to quit before the end of his last year due to an illness. He was diagnosed with consumption and died May 26, 1870. Thomas Vincent, Collet's last surviving child, died in 1902 at age 48.
Collet's daughter Agnes became a Carmelite nun and died in 1898. Collet's granddaughter, Irene Collet, probably the daughter of George A., also became a Carmelite nun and went by the name Sister Josephine Collet.
Collet wrote voluminously on a variety of topics but only published a few articles. In 1872, he published the Biographical Register of the 26th General Assembly of the State of Missouri. Collet's General Index to st. Louis County Archives was published in 1874. Collet compiled other indexes including the archives of the St. Louis Cathedral and the Catholic Church of St. Charles, which consisted of an alphabetical listing of early birth, marriage, and death records. He also indexed the early marriage records from Cahokia, Illinois.
Before joining the Missouri Historical Society, Collet held many positions at various employers. In 1845, Collet worked with John Halsell as book and stationary vendors, doing business as Halsell and Collet. Around 1848, Collet worked at a mustard factory called Western Spice Mills and did business as Collet and Johnson. The mill caught on fire in 1852. From approximately 1857-1860, Collet worked for T.M. Taylor and Company, (Collet's sister, Emma Sophia, was married to a Taylor), doing business as wholesale liquor dealers. In 1869, Collet was fired from his post as superintendent and manager of Woodward and Garesche. Around 1870, Collet kept the books for Fitzgibbon at the County Court.
The year 1875 began Collet's career with the Missouri Historical Society. He held various positions and served on many committees during his interval with the society. He was Treasurer from 1879-1883, Acting Secretary from 1881-1883, and Recording Secretary from 1888-1889. He was a member of the Executive Committee on Archeology. Throughout the 1880's, Collet was responsible for the growth of the Missouri Historical Society's museum collection of archaeological artifacts. Collet was especially interested in Indian artifacts and the Society's collection was considered to be one of the best collections in the country at the time.
The year of 1893 marked the end of Collet's affiliation with the Society. This was due to a misunderstanding between Collet and the other members of the Society. Collet wanted to sell the Society's property at 1600 Lucas Place and the newer members of the Society disagreed with him. The argument ended with Collet's resignation and a total cessation of his dealings with the Society.
Collet died on July 23, 1904 at the age of 83. He had been ill and practically an invalid the last two years of his life. He died at St. John's hospital due to the infirmities of old age. St. Alphonsus' Church held the funeral. His wife, Agnes, outlived him until 1911.