| Oakes (58,153) | Oakley (67,385) | Oaks (29,494) |
| Oates (35,589) | Oats (10,859) | Ober (16,005) |
| Oberg (19,529) | Oberts (22,522) | Obinson (41,859) |
| Obrian (19,896) | Obrien (362,175) | O'Brien (177,501) |
| Obryan (14,662) | Ocampo (35,719) | Ocasio (19,883) |
| Ochoa (140,700) | Ochs (18,609) | Oconnell (97,970) |
| O'Connell (52,325) | Oconner (35,345) | O'Conner (14,720) |
| Oconnor (196,027) | O'Connor (93,603) | Odell (116,703) |
| O'Dell (13,929) | Oden (27,828) | Odle (12,143) |
| Odom (102,930) | Odonnell (115,345) | O'Donnell (57,186) |
| Odriguez (10,923) | Odum (13,862) | Ogan (18,713) |
| Ogden (83,287) | Ogers (21,315) | Ogg (10,267) |
| Ogle (55,687) | Oglesby (37,783) | Ogletree (11,400) |
| Ogrady (14,913) | Ohara (54,802) | O'Hara (25,191) |
| Ohare (11,814) | Ohnson (16,265) | Ojeda (36,639) |
| O'Johnson (12,319) | Okeefe (42,925) | O'Keefe (22,268) |
| Olden (10,934) | Oldfield (10,476) | Oldham (50,823) |
| Olds (30,276) | Oleary (55,895) | O'Leary (29,015) |
| Oleman (12,515) | Oleson (56,042) | Olguin (19,792) |
| Olin (18,784) | Olinger (18,419) | Oliphant (17,148) |
| Oliva (33,575) | Olivares (39,737) | Olivarez (17,419) |
| Olivas (33,615) | Olive (20,223) | Oliveira (40,206) |
| Oliver (486,763) | Olivera (18,007) | Olivier (11,506) |
| Olivo (16,328) | Ollins (20,849) | Olmos (12,897) |
| Olmstead (35,323) | Olmsted (15,121) | Olney (13,393) |
| Olsen (232,489) | Olson (700,625) | Olszewski (12,839) |
| Olvera (43,345) | Omalley (38,208) | O'Malley (19,301) |
| Oman (19,911) | Omara (10,369) | Omero (10,690) |
| Omez (32,363) | Oneal (145,602) | O'Neal (33,848) |
| Oneil (116,540) | O'Neil (53,600) | Oneill (123,138) |
| O'Neill (52,517) | Ones (210,992) | Oney (12,456) |
| Ong (89,532) | Ontiveros (23,842) | Ontreras (10,129) |
| Onzales (19,039) | Onzalez (10,945) | Orales (41,287) |
