Father:  James O'NAN of Culpeper Co., VA
Mother: 
or
Father:  Dennis O'NAN
Mother:  Rebecca CLEMPSON

— (presumably) born in Shelby [now Henry(?)] Co., KY:
8.  Peter O'NAN, b. 1794, KY; m. Ellis BREWER
9.  James O'NAN, b. 1797, KY; d. 1846, Daviess Co., KY; m. 22 Aug 1823, Franklin Co., KY, Elizabeth BOWLEN

1.  Elise Greenup Jourdan.  1997.  "Births, Marriages, and Deaths of Trinity Parish, Charles County, Maryland, 1729-1797."  Pages 1-21 in Colonial Records of Southern Maryland.  Family Line Publ. (now Willow Bend Books), Westminster, MD (Broderbund CD-178).  On p. 4:

Carico, Mary Ann, d/o Abel and Elisabeth, b. 4 Nov 1748

The article says "births," but I wonder if this isn't a baptism.

2.  1790 Census.  VA Census destroyed; KY Census not taken (not yet a state).

3.  Census Index:  U.S. Selected Counties, 1800 (Broderbund CD-312):

1800 Tax List

Evan, William

KY

Henry Co.

There was no William O'Nan anywhere on this CD, but "Evan" could be a "mishearing" or a misreading of "Onan."  I checked every William in Henry Co. and this is the only one that comes close.

4.  Census Index:  U.S. Selected Counties, 1810 (Broderbund CD-313).  William O'Nan not found, but he is now in his 50s and Mary Ann is in her 60s.   Perhaps they're living with one of their children, but I have checked the households of what children I could find and none had a senior couple living with them.  I really wonder if they're still alive.  I've seen no source for their death dates.

5.  Census Index:  U.S. Selected Counties, 1820 (Broderbund CD-314).  William O'Nan not found as head-of-household.

6.  Census Index:  U.S. Selected Counties, 1830 (Broderbund CD-315).  Neither William O'Nan nor Mary O'Nan found as head-of-household.

7.  Jordan R. Dodd, ed.  1993.  Kentucky Marriages:  Early to 1850.  Liahona Research, Orem, UT (Broderbund CD-229):

Onan, Dennis	Brewer, Lidy	24 Oct 1802	Henry Co.
Onan, Clempson	Lecompt, Libey [Mrs]	12 Sep 1803	Henry Co.
Onan, Mary	Brewer, William	9 Mar 1807	Shelby Co.
Onan, Thomas	Penn, Elizabeth	29 Nov 1810	Shelby Co.
Onan, William	Brewer, Charity	26 Feb 1811	Henry Co.
Onan, Rebecca	Simmons, John	3 Feb 1814	Henry Co.
Onan, Peter	Brewer, Elis	12 Feb 1814	Henry Co.
Onan, Elizabeth [Mrs]	Griffith, Joshua	12 Sep 1816	Shelby Co.
Onan, James	Bowlen, Betsey	22 Aug 1823	Franklin Co.

Has Thomas died?  Is his widow remarrying?

8.  Homer E. Carrico.  1951.  "The Carrico Family." Filson Club Historical Quarterly 25:  217-252 (republ. 1991 as pp. 215-250 in Genealogy of Kentucky Families. Genealogical Publ. Co., Baltimore, MD; in 1997 as Broderbund CD-185).  [This article is known to contain errors; consult with caution.]  Father of William O'Nan said to be James O'Nan.

9.  Query in the 1964 Maryland Genealogical Society Bulletin, v. 5, n. 4, p. 62 (Broderbund CD-208):

O'NAN, CARRICO, SEMMES (SIMMS), CLARKSON.  Want info. on William O'NAN, b. 1754 Culpeper Co., Va., m. Mary Ann CARRICO in 1777.  She was b. 1757 in Charles Co. Md.  Who were William's parents?  Abel CARRICO the f. of Mary Ann, d. abt. 1790 in Va.  He m. abt. 1730 in Charles Co. Md. Elizabeth SIMMS, dau. of Marmaduke SIMMS (SEMMS) and Elizabeth CLARKSON.  Marmaduke SEMMS d. 1717 Charles Co. Md. His father (also named Marmaduke) d. 1692 in St. Mary's Co. Md...

10.  Descendants of Peter Carricoe I by Al Beard.

11.  WorldConnect / Ancestry World Trees (online at RootsWeb.com/Ancestry.com).

12.  LDS.  Family Search: Internet Genealogy Service:  AF - Ancestral File (online at FamilySearch.org).