- Obstetricians & Gynecologists, Waterville - Ob-Gyn Doctors, Physicians & Surgeons (Obstetrics & Gynecology)
- Occupational Health & Safety, Waterville
- Occupational Health Care Professionals, Waterville
- Off Road Vehicles, Waterville
- Off Road Vehicles Parts & Accessories, Waterville
- Off Track Betting, Waterville
- Office Planning & Design, Waterville
- Office Supplies, Equipment & Office Furniture Stores, Waterville
- Oil & Petroleum Products, Waterville - Merchant, Wholesalers, etc
- Oil Burners Services, Waterville
- Oil Change & Lubrication, Waterville - Auto Oil & Quick Lube
- Old Age Homes Service, Waterville
- Ophthalmologists, Waterville - Eye Doctors, Physicians & Surgeons (Ophthalmology)
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- Optical Stores - Eyeglasses, Lenses & Sunglasses, Waterville
- Opticians, Waterville
- Optometrists, Waterville - Eye Doctors, Physicians & Surgeons (Optometry)
- Ornamental Nursery, Waterville
- Ornamental Nursery Services, Waterville
- Orthodontist, Waterville - Teeth Straightening, Invisalign & Dental Braces
- Orthopedic & Prosthetic Appliances & Shoes, Waterville - Orthopedic Shoes
- Orthotics and Prosthetics, Waterville
- Other Building Materials & Supplies, Waterville
- Other Lodging, Waterville
- Outdoor Furniture & Patio Furniture, Waterville - Garden Furniture Stores
- Outpatient Services, Waterville - Outpatient Care
- Oxygen & Respiratory Equipment & Supplies, Waterville - Oxygen Concentrator, Tank, Machine, etc
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