Entries are transcribed as written in the original, be sure to check for spelling variations. Also check below the surname list for additional records where it is suspected that the enumerator may have written the name incorrectly.
| Surname | Count |
|---|---|
| O'Brien | 1 |
| O'Dea | 1 |
| O'Donell | 5 |
| O'Donnell | 1 |
| O'Flynn | 2 |
| O'Rourke | 3 |
| O Brien | 3 |
| O Connor | 1 |
| O Hanley | 1 |
| Oakley | 4 |
| Oar | 1 |
| Oats | 1 |
| Oberne | 5 |
| Obright | 7 |
| Odbard | 13 |
| Odlum | 8 |
| Oehrn | 5 |
| Offord | 1 |
| Ogden | 5 |
| Ogilvie | 2 |
| Ogilvy | 5 |
| Old | 3 |
| Olish | 4 |
| Oliver | 31 |
| Olliver | 6 |
| OLoughlin | 5 |
| Oman | 9 |
| Omstead | 1 |
| Oneill | 1 |
| Onyon | 1 |
| Orme | 1 |
| Ormsby | 5 |
| Oromston | 1 |
| ORorke | 1 |
| ORourke | 5 |
| Orr | 3 |
| Orth | 1 |
| Oruna(?) | 4 |
| Osborn | 19 |
| Osgood | 6 |
| Ostrander | 2 |
| Ostrom | 4 |
| Otterbein | 5 |
| Otto | 39 |
| Otton | 4 |
| Ottwein | 1 |
| Overholt | 7 |
| Owen | 2 |
| Owens | 12 |
| Ozborne | 2 |
No alternate surnames starting with O in the selected area.
The following surnames have been suggested as corrections of the transcribed surname:
| Suggested Surname | Transcribed Surname |
|---|---|
| Oehm | Oehrn |
| Orum | Oruna(?) |