- Operating of Conference and Exhibition Centres
- Oats Growing
- Obstetrics and gynaecology services
- Occasional Retail Sale of Home Furnishings and Appliances
- Occasional Retail Sale of Clothing
- Occupational health and safety organisations
- Occupational therapy services
- Occupational Training and Certification
- Office Buildings
- Office Cleaning Services
- Office Parks
- Office Supplies and Equipment
- Office supplies and stationery stores
- Offshore Exploration and Production
- Offshore Platform Construction and Maintenance
- Oil and gas exploration services
- Oil and Gas Proppant Supply
- Oil and Gas Well Logging and Perforation Services
- Old-age pensions
- On-Demand Freight Air Transport
- On-Demand Video Services
- On-site lodging facilities
- Onboarding & Orientation Programs
- Oncology
- Online Agents
- Online Auctions
- Online Banks
- Online bidding services
- Online content and e-book publishing
- Online content/blogging services
- Online Content and Digital Media publishing
- Online/Distance Learning Programs
- Online education
- Online Fish Retailers
- Online Food Ordering Platforms
- Online Forums and Message Boards
- Online Gambling Sites
- Online information services
- Online Learning Platforms
- Online Learning Platforms and Support
- Online learning programs
- Online media material delivery
- Online retail sales of musical instruments and scores
- Online retail sale of fresh, dried, and artificial flowers
- Online retail sale of antiques
- Online retail sale of hearing aids
- Online Retail Sales
- Online search portals and information retrieval systems
- Online Shopping
- Online Streaming Service Subscription
- Onshore Exploration and Production
- Operating ostrich farms
- Operation and Management of Motor Racing Events
- Operation, leasing and sale or rental of spacecrafts
- Operation of air cargo warehouses
- Operation of Passenger Information Systems (PIS) at Railway Stations
- Operational Auditing
- Ophthalmology
- Optics and Scopes
- Optometry services
- Oral and maxillofacial surgery
- Organic Egg Production
- Organic food producers and distributors
- Organic Fruit Processing
- Organic vegetable growing
- Organizing strikes and other forms of protest
- Ornamental Fish Breeding
- Orthopedic Boot Modification
- Orthopedic doctors
- Orthopedics
- Orthopedics & Sports Medicine
- Other Animal Processing (Horse, Rabbit, etc.)
- Other Business Support Activities
- Other holding company activities n.e.c. (not elsewhere classified)
- Other information technology consulting services
- Other manufacture special purpose machines n.e.c.
- Other miscellaneous specialised construction works n.e.c.
- Other non-profit organisations
- Other professional, scientific, technical consulting services
- Other wholesale trade in intermediate goods
- Other wholesale trade in specialized or auxiliary machine tools
- Out-of-home (OOH) advertising activities
- Outdoor activities
- Outdoor Activities and Ziplining
- Outdoor Activities and Excursions
- Outdoor Adventure Activities
- Outdoor Adventure Expeditions
- Outdoor Advertising
- Outdoor fitness centers
- Outdoor Gear & Equipment Rental
- Outdoor/Picnic Catering
- Outdoor Recreation Education
- Outdoor Sports Facilities Operation
- Outfitting and Guiding Activities
- Outpatient/ambulatory care centers
- Outpatient Care Services
- Outpatient Medical Services
- Outpatient Mental Health Services
- Over-the-counter Medicines
- Operating of own-account professional, scientific & technical service activities n.e.c.
- Operation of warehousing facilities for water transport activities