1) Both Onan and O'Nan are found in research and exist today in use. I try to stay with source spelling unless it confuses sorting. I will use Onan as default spelling. It is the way my name is spelled from earliest records.
Here is an interesting discussion of the subject from another source. In TREASURE UP THE MEMORY by James Frederick O'Nan in 1969, he says that O'Nan is an affectation of Onan. He says, "The writer's father, grandfather and great-grandfather all spelled the name, without variation, Onan. In a recent letter from Mr. Cary W. O'Nan he notes; 'In most of the records I have search, including Court House Records, and in my memory the name was always spelled Onan, until about the end of World War I, when the change in spelling to O'Nan began to take place. I always spelled my name Onan until about 1922, before I started spelling it O'Nan.'"
2) Possible French Connection
Having never found evidence of Onan's on both sides of the Atlantic which can be lineage linked, a research assistant suggested the name originated as French. It is at least a working hypothesis which cannot be ignored.
The surname ONAN could derive from the French name AUNAN or AUNANT. The English phonetic pronunciation of the French AUNAN(T) is ONAN.. Actual evidence of the event occurs in the Registers of the French Church, Threadneedle Street, London. On the same line is the child Marie Onan, baptized daughter of Pierre Aunant and Elizabeth Soyer. The documentation presumably made by an Englishman.
The Protestant church had been legally tolerated in France under the Edict of Nantes authorized by king Henry IV in 1598. The Edict was withdrawn by king Louis XIV in 1685 declaring France entirely Roman Catholic. The Protestant (Calvinist) French, known as Huguenots, were no longer religiously protected and many fled for their lives to other countries.
There seems to be two groups of Onans. One is the result of coming from France through England in the late 1600's. The first Aunant I have on record is Jean Aunan(t) who married Cibille Dumas in Nimes, France on November 2, 1631. They had 8 children. Next I have Jean Aunant of Nimes, son of Jean Aunant and Sibelle Dumas, and wife Marie Soyer; listed as seeking naturalization in Carolina in 1695,96. The path of this family of French refugees in the colonies and later is told by James Frederick O'Nan in his book "Treasure Up The Memories", Cincinnati OH, 1970.
The second group, of which I am a member and this genealogy is all about, seems to have come from the British Isles about 1807. James and Mary (Collins) Onan raised a family of 8 children. They lived in the area of Ithaca, NY and also Cattaraugus County, NY from about 1825 to 1870. They were farmers and lumbermen. James father was Alexander. James, Mary and two children are buried in a cemetery near Fitch, Cattaraugus County, NY. Other children started the move west to Illinois and Wisconsin as early as 1850. Those to Minnesota and Iowa, later. James's oldest son, Warren, my great-great grandfather, moved to Moorhead MN in 1875.
3) At this time I can make no connection with the early Carolina and later New York families. A long and faithfull correspondant, Beth Onan Hugenot, found her roots in the Carolina O'Nan's We struggled together to find a lineag link but she died, in her words, a "wanabe cousin".
4) Was ther another name change?
The early immigrants volunteered to census takers they were born in Scotland or Ireland. We can't find them there. But, my belief is they had no reason to change their name when they emigrated, and did not. This leads to one other ethinic caution. Sometime after the Protestand Reformation of 1517 my Onan's became Protestant. When looking for them in religiously divided countries or whan population shifts were made, (The Ulster Plantation), the reseacher should look for Protestants.
It is my great hope that others will find the answers.
From BIOG0007 ONAN A one page summary
Onan from Aunan(t) Huguenots Two immigrations
Revised: 6 May 1998
Rev. Mar 2013
David Warren Onan II
linked from 4