- Obstetricians & Gynecologists, North Wales - Ob-Gyn Doctors, Physicians & Surgeons (Obstetrics & Gynecology)
- Occupational Health Care Professionals, North Wales
- Occupational Therapy, North Wales - Occupational Health Specialists
- Office Supplies, Equipment & Office Furniture Stores, North Wales
- Oil & Petroleum Products, North Wales - Merchant, Wholesalers, etc
- Oil Change & Lubrication, North Wales - Auto Oil & Quick Lube
- Ophthalmic Goods, North Wales
- Ophthalmic Instruments & Machinery, North Wales
- Ophthalmologists, North Wales - Eye Doctors, Physicians & Surgeons (Ophthalmology)
- Optical Stores - Eyeglasses, Lenses & Sunglasses, North Wales
- Opticians, North Wales
- Optometrists, North Wales - Eye Doctors, Physicians & Surgeons (Optometry)
- Organizing Services & Products – Household & Business, North Wales
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- Oriental Rugs, North Wales
- Ornamental Greenhouses, North Wales
- Ornamental Nursery, North Wales
- Ornamental Nursery Services, North Wales
- Orthodontist, North Wales - Teeth Straightening, Invisalign & Dental Braces
- Other Building Materials & Supplies, North Wales
- Other Lodging, North Wales
- Outdoor Equipment & Accessories, North Wales
- Outdoor Furniture, North Wales
- Outdoor Furniture & Patio Furniture, North Wales - Garden Furniture Stores
- Outerwear Stores, North Wales - Jackets, Sunglasses, Hats, Caps, Coats, Gloves & More
- Ownership Organizations, North Wales
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