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 | 8 |
| O'Conner | 1 |
| O'Connor | 9 |
| O'Glynn | 2 |
| O'Halloran | 2 |
| O'Neil | 1 |
| O'Ray | 6 |
| O'Regan | 1 |
| O'Reilly | 1 |
| O'Sullivan | 6 |
| Oakes | 12 |
| Oakley | 5 |
| Obrien | 1 |
| Obrin | 2 |
| OBrine | 1 |
| Oconnell | 5 |
| Oconnor | 9 |
| ODanahaye | 2 |
| Oddie | 6 |
| Odell | 2 |
| Odie | 1 |
| Odonnell | 1 |
| ODonohoe | 5 |
| Ogden | 1 |
| Ogilvie | 1 |
| OGrady | 7 |
| Ohallerain | 6 |
| Oheru | 1 |
| OLearly | 5 |
| OLeary | 14 |
| Oliver | 51 |
| Omallay | 5 |
| Oneil | 7 |
| Oneill | 8 |
| Onyon | 6 |
| OReilly | 16 |
| ORielly | 2 |
| Oriley | 1 |
| Orishan | 7 |
| Orminson | 1 |
| Orourke | 1 |
| Orr | 19 |
| Orsborne | 4 |
| Orser | 7 |
| Osborn | 1 |
| Osborne | 9 |
| Oshea | 7 |
| Osterhouf | 4 |
| Osterhout | 16 |
| Ostrander | 4 |
| Ostrem | 2 |
| Ostrom | 3 |
| OSullivan | 15 |
| Oulton | 14 |
| Outraeuduk | 6 |
| Outram | 2 |
| Oval | 1 |
| Overbaugh | 7 |
| Owen | 6 |
| Owens | 19 |
The following surnames have suggested "corrections" for what was recorded in the original by the enumerator:
| Suggested Surname |
|---|
| ODonahaye |
| OLeary |
| Orichan |
| Osborne |
The following surnames have been suggested as corrections of the transcribed surname:
| Suggested Surname | Transcribed Surname |
|---|---|
| Ostrence | Ostrem |