- Obstetricians & Gynecologists, Plainville - Ob-Gyn Doctors, Physicians & Surgeons (Obstetrics & Gynecology)
- Occupational Health & Safety, Plainville
- Occupational Health Care Professionals, Plainville
- Off Road Vehicles, Plainville
- Off Road Vehicles Parts & Accessories, Plainville
- Office Supplies, Equipment & Office Furniture Stores, Plainville
- Oil Burners Services, Plainville
- Oil Change & Lubrication, Plainville - Auto Oil & Quick Lube
- Oil Field Water Purification & Filtration Equipment, Plainville
- Oil Industry & Oil Field Equipment & Services, Plainville - Oil Companies
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- Optical Stores - Eyeglasses, Lenses & Sunglasses, Plainville
- Opticians, Plainville
- Optometrists, Plainville - Eye Doctors, Physicians & Surgeons (Optometry)
- Ornamental Iron - Residential, Plainville - Wrought Iron Work Supplies, Parts, Gates, Railings, Stairs, Spirals, Fence, etc
- Ornamental Metal Work, Plainville - Steel, Iron, Bronze, etc
- Ornamental Nursery, Plainville
- Ornamental Nursery Services, Plainville
- Orthodontist, Plainville - Teeth Straightening, Invisalign & Dental Braces
- Other Building Materials & Supplies, Plainville
- Other Electrical Products, Plainville
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