| Surname | Country | State | County or Province | From | To | Member |
|---|---|---|---|---|---|---|
| O'Brien | Ireland | Waterford | H011-1 | |||
| Ochocki | Poland | Grudziadz | 1700 | present | S017-1 | |
| O'Keefe | Ireland | 1875 | Present | T011-2 | ||
| O'Leary | T013-1 | |||||
| Oleson | Norway | Telemark | Bamble | 1818 | present | O001-1 |
| Oleson | USA | Wisconsin | Dane County | 1818 | present | O001-1 |
| Oleson | USA | Minnesota | Hennepin County | 1818 | present | O001-1 |
| Oleson | Norway | Akerhus | Kongsvinger | 1834 | 1952 | O001-1 |
| Oleson | USA | Minnesota | Murray County | 1834 | 1952 | O001-1 |
| Oleson | USA | North Dakota | Stark County | 1834 | 1952 | O001-1 |
| Oliphant | USA | Maryland | Wicomico County | 1720 | 1793 | S009-1 |
| Oliphant | USA | North Carolina | Surry County | 1793 | 1828 | S009-1 |
| Oliphant | USA | Indiana | Monroe County | 1828 | present | S009-1 |
| Oliver | USA | Virginia | 1810 | J009-1 | ||
| Oliver | USA | Alabama | 1832 | 1870 | J009-1 | |
| Olmsted | W019-1 | |||||
| Olsen | A010-1 | |||||
| Olson | Sweden | bef 1891 | B019-1 | |||
| Olson | Sweden | (Name changed each generation) | bef 1891 | B019-1 | ||
| Olson | Norway | Stavanger | Rogalund | 1860 | J005-1 | |
| Olson | Norway | Stavanger | Rogalund | 1700 | 1900 | J005-2 |
| Olson | Sweden | K019-1 | ||||
| Olson | Norway | Telemark | Bamble | 1818 | present | O001-1 |
| Olson | USA | Illinois | Cook County | 1891 | 1973 | B019-1 |
| Olson | USA | Minnesota | Otter Tail County | 1900 | J005-1 | |
| Olson | USA | Wisconsin | Dane County | 1854 | present | O001-1 |
| Olson | Canada | Saskatchewan | 1907 | 1926 | B019-1 | |
| Olson | USA | Minnesota | Hennepin County | 1895 | present | O001-1 |
| Olson | USA | New York | Chautauqua County | 1940 | 1957 | B019-1 |
| Olson | Norway | Akerhus | Kongsvinger | 1834 | 1952 | O001-1 |
| Olson | USA | Minnesota | Murray County | 1834 | 1952 | O001-1 |
| Olson | USA | North Dakota | Stark County | 1834 | 1952 | O001-1 |
| Olson | USA | Minnesota | Murray County | 1834 | 1952 | O001-1 |
| O'Meara | Ireland | M028-1 | ||||
| O'Meara | USA | Wisconsin | ||||
| O'Neil | Ireland | G015-1 | ||||
| O'Neil | Ireland | County Kilkenny | O004-1 | |||
| O'Neil | Ireland | W012-1 | ||||
| O'Neil | USA | Iowa | W012-1 | |||
| O'Neil | USA | Nebraska | W012-1 | |||
| O'Neill | Ireland | County Galway | Ross Parish | 1800 | 1950 | O006-1 |
| O'Neill | Ireland | County Galway | Maam | 1800 | 1950 | O006-1 |
| O'Neill | Ireland | County Tyrone | Ross Parish | 1200 | 1800 | O006-1 |
| O'Neill | Ireland | Ballyheragh | County Mayo | M014-1 | ||
| Ordnung | Germany | E005-1 | ||||
| Ordnung | Prussia | E005-1 | ||||
| Ortwine | Germany | M026-1 | ||||
| Ortwine | USA | Michigan | M026-1 | |||
| Osiecki | Russia/Poland | Karnkowo Parish | Bydgoszcz Province | 1805 | ? | R003-1 |
| Ostberg | Sweden | Falo Christine | Dalarne | G015-1 | ||
| Ostermeier | Prussia | Westfalia | E005-1 | |||
| Ostermeier | USA | Nebraska | E005-1 | |||
| Ostermeier | USA | Missouri | 1853 | 1865 | E005-1 | |
| Ostermeier | USA | Illinois | 1865 | Present | E005-1 | |
| Ostermeyer | E005-1 | |||||
| Outten | USA | Ohio | Madison County | 1800 | 1840 | K008-1 |
| Outten | USA | Ohio | Fayette County | 1800 | 1840 | K008-1 |
| Outten | USA | Ohio | Ross County | 1800 | 1840 | K008-1 |
| Outten | USA | Ohio | Pickaway County | 1800 | 1840 | K008-1 |
| Owen | USA | Virginia | 1750s | present | M011-1 | |
| Owen | USA | Georgia | 1750s | present | M011-1 | |
| Owen | USA | Kentucky | 1750s | present | M011-1 | |
| Owen | USA | Missouri | 1750s | present | M011-1 | |
| Oyler | USA | North Carolina | 1790 | A007-1 | ||
| Oyler | USA | Ohio | 1790 | A007-1 | ||
| Oyler | USA | Kentucky | 1790 | A007-1 | ||
| Oyler | USA | Indiana | 1790 | A007-1 |