- O'Brien
 - Formed to promote the interests of all O'Briens who are descended from their progenitor, Brian Boru. - O'Dea
- Also spelled: O'Day, Dee, or Day, includes family history, current news, and genealogical queries.
- O'Hearn
- Dedicated to the exchange of genealogical information from people around the globe searching the surname O'Hearn.
- O'Kelly
- Research your Irish roots and discover Irish genealogical links.
- O'Mahony
- With membership and event information.
- Odden
- Gedcom files, pictures, and maps of family genealogy.
- Ogle / Tasker / Ridout / Tayman
- Also Tayloe and Carter. Traces the genealogy of these Maryland, Virginia, and Washington D.C. families.
- Oglesby
- Also Anderson, Barker, Carpenter, Crawford, Druehl, Ellis, Hite, Kinzel, Lawhon, Meriwether, Naumann/Norman, Thornton and Yundt.
- Olive
- Genealogy, history, reunions, coat of arms.
- Oliver
- With family tree and some photos.
- Olmütz
 OLochlainns Irish Families
- Home of the Irish Genealogical Foundation and Olochlainns Irish Family Journal, free listing and bulletin board online.- Olree
- Also including: Evans, Hijman, Hymen, McCoy, Parker, Kislowski, and Malasinski.
- Olzheim
- In Dutch.
- Openshaw (1)
- Oravecz
- Dedicated to families with variants of this surname. Including Oravec, Oravetz and others.
- Ortenburger and Hackstock
- Family history in the USA, Germany, Silesia, and the Russian Empire.
- Orth
- Family history of Johann Peter Orth, born 1818, in Furstengrund, Hessen, Germany, and died in 1893 in Wallace Township, Perth County, Ontario, Canada.
- Oshel
- Oster
- Also including: Galliano, Kennedy, and Hickman families.
 Oulton
- Also Dyke, Winnard, Sinclaire, Rigby, Raper, Crook, and Jolley.- Overend
- Owens
- Tracing the descendants of Nathaniel Owens and Rebecca Massey.
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