| Family Name | Source of Relationship |
|---|---|
| O'Brien/Brien | My various Brien/O'Brien family connections. |
| O'Callaghan/Callaghan | Related to me through at least two DNA connections. |
| O'Connell/Connell | Related to me through at least one DNA connection, maybe more. |
| O'Dea | Related through John and Margaret (nee Murphy) McCabe descendants. |
| O'Donnell | My various O'Donnell family connections. |
| O'Hare/O'Hear/O'Hara | Related through my Hearty/McCabe family connection in the Dunleer area of Co. Louth. |
| O'Kane | Related via several O'Kane descendants with DNA matches to me. |
| O'Keefe/Keefe | My O'Keefe/Keefe family connections, including the family of my paternal grandmother, Gertrude A. O'Keefe. |
| O'Regan/Regan | Ancestors of my DNA cousin Josephine Leonie Rowe, in Australia. |
| O'Reilly/Reilly | There are currently two known O'Reilly/Reilly families that have connections to me. |
| Owen | Owen family of Scotland with 1 direct DNA connection to me and multiple shared matches thru Russell Bartz. |