- Office & Meeting Equipment & Supplies Rental, Welland
- Office Furniture & Equipment Rental & Leasing, Welland
- Office Supplies, Equipment & Office Furniture Stores, Welland
- Oil & Gas Companies, Welland
- Oil And Petroleum Products, Welland
- Oil Bulk Stations, Welland
- Oil Change & Lubrication, Welland - Auto Oil & Quick Lube
- Oil Industry & Oil Field Equipment & Services, Welland - Oil Companies
- Optical Stores - Eyeglasses, Lenses & Sunglasses, Welland
- Opticians, Welland
- Optometrists, Welland - Eye Doctors, Physicians & Surgeons (Optometry)
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- Optometry Clinics, Welland - Eye Clinics
- Ornamental Nursery, Welland
- Ornamental Nursery Consultants, Welland
- Ornamental Nursery Services, Welland
- Orthopedic & Prosthetic Appliances & Shoes, Welland - Orthopedic Shoes
- Orthopedic Appliances, Welland
- Other Building Materials & Supplies, Welland
- Outdoor Advertising & Billboard, Welland
- Outdoor Equipment & Accessories, Welland
- Outpatient Services, Welland - Outpatient Care
- Overhead Door, Welland
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