Occupational Title | Brief Number | HOC Code |
| Obstetricians (gynecologists) | 578 | IRS |
| Occupational Health Nurses (registered professional) | 75 | IES |
| Occupational Social Workers | 84 | SEA |
| Occupational Therapists | 74 | SRE |
| Occupational Therapy Aides (therapists) | 74 | SEA |
| Occupational Therapy Assistants (therapists) | 74 | SCE |
| Ocean Engineers | 163 | IRE |
| Ocean Engineers | 200 | IRE, IRS |
| Oceanographers | 200 | IRE, IRS |
| Oceanographers, Biological | 200 | IRE |
| Oceanographers, Chemical | 200 | IRS |
| Oceanographers, Geological | 200 | IRE, IRS |
| Oceanographers (geologists) | 129 | IRE |
| Oceanographers (geophysicists) | 148 | IRS |
| Oceanographers, Physical | 200 | IRE, IRS |
| Oceanographic Engineers | 200 | IRE, IRS |
| Oceanographic Engineers | 163 | IRE |
| Ocean Physicists | 200 | IRE, IRS |
| Offal Separators (slaughtering & meat-packing production) | 423 | REC |
| Office Clerks | 3 | no code |
| Office Machine Technicians | 90 | RES |
| Office Managers (congressional aides) | 525 | ESR |
| Office Nurses (registered professional) | 75 | SIR |
| Officers (armed forces) | 439 | no code |
| Officers, Canine Enforcement | 412 | CEI |
| Officers, CBP | 412 | CEI |
| Officers, Customs and Border Protection | 412 | CEI |
| Officers, Equal Employment Opportunity (human resources) | 258 | SRI |
| Oil Dispatchers (gas & oil distribution) | 196 | ESR |
| Old World Archaeologists | 343 | IRE |
| Ombudsman | 150 | SCR |
| Oncologists, Gynecologic (obstetrician-gynecologists) | 578 | IRS |
| Oncologists, Radiation | 462 | IRS |
| Online Merchants (entrepreneurs) | 550 | ESA, ESR |
| Open-Source Intelligence Workers | 626 | AES, ERS |
| Operating Engineers | 171 | RIS |
| Operations Research Analysts | 514 | IRE |
| Operators-in-Training (water & wastewater treatment plant) | 517 | REI |
| Ophthalmic Assistants | 680 | CIS |
| Ophthalmic Laboratory Technicians | 471 | no code |
| Ophthalmic Medical Assistants | 5 | CIS |
| Ophthalmic Medical Personnel | 680 | CIS |
| Ophthalmic Technicians | 680 | CIS |
| Ophthalmic Technologists | 680 | CIS |
| Ophthalmologists | 13 | ISR |
| Opticians, Dispensing | 123 | RIS |
| Optometric Assistants | 229 | SCI |
| Optometric Technicians | 229 | SCI |
| Optometrists | 85 | ISE |
| Oral Interpreters | 365 | SCE |
| Oral and Maxillofacial Pathologists (dentists) | 59 | IRE |
| Oral and Maxillofacial Radiologists (dentists) | 59 | IRS, ISR |
| Oral and Maxillofacial Surgeons (dentists) | 59 | IRE |
| Oral Transliterators | 365 | SCE |
| Order Clerks (office) | 3 | CES |
| Order Fillers (stock clerks) | 108 | RCE |
| Orderlies (nursing assistants) | 407 | SER |
| Ordinary Seamen (merchant mariners) | 379 | REI |
| Organic Chemists | 153 | IRE |
| Organic Petrologists | 560 | ISR |
| Organizational Economists | 233 | IAS |
| Organization Development Specialists/Managers (training & development) | 572 | EIS, SRE |
| Orientation and Mobility Specialists | 539 | SER |
| Ornamental Blacksmiths | 588 | RIC, RIE |
| Ornamental Ironworkers (construction) | 359 | RIS |
| Orthodontic Technicians (dental) | 31 | RCE |
| Orthodontists (dentists) | 59 | IRS |
| Orthopedic Bioengineers | 519 | IRE |
| Orthopedic Surgeons | 665 | IRA |
| Orthotic Assistants | 601 | RSE |
| Orthotic Fitters | 601 | RSE |
| Orthotic and Prosthetic Assistants | 601 | RSE |
| Orthotic and Prosthetic Technicians | 601 | RIE |
| Orthotic Technicians | 601 | RIE |
| Orthotists | 469 | RSE |
| Osteopathic Physicians | 667 | ISR |
| Outfitters | 354 | RES |
| Outfitters, Boat (pleasure craft boatbuilders) | 661 | REI |
| Outreach Librarians (health sciences) | 9 | no code |
| Oven Tenders (bakery products) | 410 | RES |
| Overlock Machine Operators (knit goods industry) | 413 | CRS |
| Owners and Managers (retail jewelers) | 652 | ESC |