| Name | # Entries |
|---|---|
| O'' | 3118 |
| O'liver | 2701 |
| Osbore | 2307 |
| O '' | 2082 |
| Obrien' | 1988 |
| O Neill | 1648 |
| O''Brien | 1191 |
| O'oneill | 1074 |
| Ong Ong | 925 |
| Osulivan | 920 |
| O'oconnor | 782 |
| O Sullivan | 779 |
| O Keefe | 722 |
| O | 635 |
| O''''Brien | 628 |
| O'oneil | 615 |
| Otoole | 559 |
| Orachard | 530 |
| O Dea | 447 |
| Ogilvi | 417 |
| Oloughlin | 362 |
| O'haloran | 352 |
| O''Neill | 338 |
| O''Connor | 327 |
| Owen | 325 |
| O Brien | 319 |
| O''Keefe | 308 |
| Ooi | 289 |
| O'h | 275 |
| Obst | 271 |
| O''Sullivan | 268 |
| Oldings | 263 |
| O'odwyer | 217 |
| O''Connell | 212 |
| Ord | 201 |
| Oo | 194 |
| O Gorman | 182 |
| O''Brian | 176 |
| Os'ullivan | 166 |
| O''Shea | 154 |
| Onley | 150 |
| Obeid | 147 |
| O Leary | 146 |
| O''Toole | 145 |
| Odell | 142 |
| Olderman | 142 |
| Ozdemir | 137 |
| Orth | 136 |
| Orourke | 135 |
| Oliveri | 135 |
| O''''Connell | 129 |
| O''Leary | 129 |
| Omeara | 128 |
| O''Rourke | 127 |
| Offord | 126 |
| Ohanlon | 126 |
| Ocampo | 123 |
| Orman | 122 |
| Orme | 117 |
| Odisho | 116 |
| Olivieri | 114 |
| O''Keeffe | 113 |
| Oddy | 112 |
| O''Malley | 112 |
| O Bryan | 111 |
| Oflynn | 111 |
| Omer | 109 |
| O Shea | 109 |
| Odwyer | 107 |
| O''Halloran | 107 |
| O Dell | 98 |
| Ortiz Ortiz | 98 |
| Olding | 97 |
| O'louglin | 97 |
| Ostojic | 96 |