| Observer Helper, Gravity Prospecting | Observer Helper, Seismic Prospecting | Observer, Electrical Prospecting |
| Observer, Gravity Prospecting | Observer, Seismic Prospecting | Obstetrician |
| Occupational Analyst | Occupational Therapist | Occupational Therapy Aide |
| Occupational Therapy Assistant | Occupational-safety-and-health Inspector | Oceanographer, Assistant |
| Odd Bundle Worker | Odd-piece Checker | Odd-shoe Examiner |
| Offal Icer, Poultry | Offal Separator | Offbearer, Pipe Smoking Machine |
| Offbearer, Sewer Pipe | Office Copy Selector | Office Helper |
| Office Supervisor, Animal Hospital | Office-machine Servicer | Office-machine-service Supervisor |
| Office-machine-servicer Apprentice | Offset-duplicating-machine Operator | Offset-press Operator I |
| Offset-press Operator Ii | Offset-press-operator Apprentice | Oil Boiler |
| Oil Dipper | Oil Pumper | Oil-burner-servicer-and-installer |
| Oil-burner-servicer-and-installer Helper | Oil-field Equipment Mechanic | Oil-field Equipment Mechanic Supervisor |
| Oil-pipe Inspector | Oil-pipe-inspector Helper | Oil-recovery-unit Operator |
| Oil-seal Assembler | Oil-spot Washer | Oil-well-service Operator |
| Oil-well-service-operator Helper | Oil-well-services Supervisor | Oiler |
| Oiling-machine Operator | Oilseed-meat Presser | Olive Brine Tester |
| Oliving-machine Operator | Opaquer | Open-developer Operator |
| Opener | Opener I | Opener Ii |
| Opener Tender | Opener-verifier-packer, Customs | Operating Engineer |
| Operating-engineer Apprentice | Operating-table Assembler | Operational Test Mechanic |
| Operations And Intelligence Assistant | Operations Manager | Operations Officer |
| Operations Supervisor, Nuclear Power Plant | Operations-research Analyst | Operator, Automated Process |
| Operator, Catalyst Concentration | Operator, Prefinish | Ophthalmic Photographer |
| Ophthalmic Technician | Ophthalmologist | Optical Engineer |
| Optical-effects Layout Person | Optical-effects-camera Operator | Optical-element Coater |
| Optical-glass Etcher | Optical-glass Silverer | Optical-instrument Assembler |
| Optician | Optician Apprentice | Optician Apprentice, Dispensing |
| Optician, Dispensing | Optomechanical Technician | Optometric Assistant |
| Optometrist | Oral And Maxillofacial Surgeon | Oral Pathologist |
| Orchestrator | Order Caller | Order Clerk |
| Order Clerk, Food And Beverage | Order Department Supervisor | Order Detailer |
| Order Dispatcher, Chief | Order Filler | Order Filler, Linseed Oil |
| Order Runner | Order-control Clerk, Blood Bank | Ordering-machine Operator |
| Orderly | Ordinary Seaman | Ordnance Artificer |
| Ordnance Engineer | Ordnance Truck Installation Mechanic | Ordnance-artificer Helper |
| Organ-pipe Maker, Metal | Organ-pipe Voicer | Oriental-rug Repairer |
| Oriental-rug Stretcher | Orientation And Mobility Therapist For The Blind | Ornament Maker, Hand |
| Ornament Setter | Ornamental-iron Worker | Ornamental-iron-worker Apprentice |
| Ornamental-machine Operator | Ornamental-metal Worker | Ornamental-metal-worker Apprentice |
| Ornamental-metal-worker Helper | Ornamental-metalwork Designer | Orthodontic Band Maker |
| Orthodontic Technician | Orthodontist | Orthopedic Assistant |
| Orthopedic-boot-and-shoe Designer And Maker | Orthoptist | Orthotics Assistant |
| Orthotics Technician | Orthotist | Osteopathic Physician |
| Otolaryngologist | Outboard-motor Inspector | Outboard-motor Mechanic |
| Outboard-motor Tester | Outfitter, Cabin | Outpatient-admitting Clerk |
| Outside Cutter, Hand | Outside Property Agent | Outside-plant Engineer |
| Outsole Cutter, Automatic | Outsole Flexer | Outsole Scheduler |
| Oven Dauber | Oven Operator | Oven Operator, Automatic |
| Oven Tender | Oven-equipment Repairer | Oven-heater Helper |
| Oven-press Tender I | Oven-press Tender Ii | Overcoiler |
| Overedge Sewer | Overhauler | Overhauler Helper |
| Overhead Cleaner Maintainer | Overhead Crane Operator | Overlay Plastician |
| Overlock Sewing Machine Operator | Overlock-machine Operator, Complete Garment | Oxidized-finish Plater |
| Oxidizer | Oxygen-furnace Operator | Oxygen-plant Operator |
| Oyster Floater |
