| Orcutt (18,213) | Ord (18,294) | Ordan (10,371) |
| Ordon (10,597) | Ordonez (26,965) | Ordway (11,389) |
| Oreilly (23,468) | O'Reilly (11,631) | Orellana (39,886) |
| Organ (54,468) | Orlando (34,170) | Orman (19,803) |
| Ormsby (13,352) | Ornelas (35,009) | Orona (16,623) |
| Orosco (18,775) | Orourke (39,087) | O'Rourke (21,327) |
| Orozco (105,980) | Orr (197,549) | Orres (42,026) |
| Orris (62,825) | Orrison (12,038) | Orta (11,996) |
| Ortega (245,100) | Ortez (22,609) | Orth (26,879) |
| Ortiz (647,995) | Orton (53,841) | Osborn (178,417) |
| Osborne (228,564) | Osburn (33,365) | Osgood (34,331) |
| Oshea (21,038) | O'Shea (9,948) | Osman (15,250) |
| O'Smith (15,555) | Osorio (48,390) | Oss (28,785) |
| Osteen (13,029) | Oster (39,550) | Osterman (9,933) |
| Ostrander (38,911) | Ostrom (11,316) | Ostrowski (16,117) |
| Osullivan (23,691) | O'Sullivan (12,281) | Osuna (11,966) |
| Oswald (51,123) | Oswalt (15,521) | Otero (45,907) |
| Otis (37,785) | Oto (11,761) | Otoole (22,824) |
| O'Toole (13,390) | Ott (128,141) | Otte (12,392) |
| Otten (13,851) | Otto (96,532) | Ouellette (32,569) |
| Oung (20,298) | Ousley (11,245) | Outlaw (23,630) |
| Overby (18,920) | Overman (18,770) | Overstreet (42,418) |
| Overton (68,769) | Oviedo (10,187) | Owell (12,208) |
| Owen (323,875) | Owens (721,236) | O'Williams (9,961) |
| Owings (16,733) | Own (10,629) | Oxendine (12,523) |
| Oxford (13,725) | Oxley (14,055) | Oyer (12,579) |
