- Occupational Health & Safety, Wallingford
- Occupational Health Care Professionals, Wallingford
- Office Supplies, Equipment & Office Furniture Stores, Wallingford
- Oil & Gas Exploration & Development, Wallingford - Oil Field Exploration, Monitoring & Remediation
- Oil Change & Lubrication, Wallingford - Auto Oil & Quick Lube
- Oil Industry & Oil Field Equipment & Services, Wallingford - Oil Companies
- Ophthalmologists, Wallingford - Eye Doctors, Physicians & Surgeons (Ophthalmology)
- Optical Stores - Eyeglasses, Lenses & Sunglasses, Wallingford
- Opticians, Wallingford
- Optometrists, Wallingford - Eye Doctors, Physicians & Surgeons (Optometry)
- Oriental Restaurants, Wallingford
- Ornamental Nursery, Wallingford
- Ornamental Nursery Products, Wallingford
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- Ornamental Nursery Services, Wallingford
- Orthopedic & Prosthetic Appliances & Shoes, Wallingford - Orthopedic Shoes
- Orthopedists, Wallingford - Orthopedic Surgery Doctors, Physicians & Surgeons
- Other Building Materials & Supplies, Wallingford
- Other Electrical Products, Wallingford
- Other Industrial Contractors, Wallingford
- Other Specialties, Wallingford - Doctors, Physicians & Surgeons
- Outdoor & Lawn & Garden Equipment Repair & Service, Wallingford
- Outdoor Advertising & Billboard, Wallingford
- Outdoor Equipment & Accessories, Wallingford
- Outpatient Services, Wallingford - Outpatient Care
- Ownership Organizations, Wallingford
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