- Obstetricians & Gynecologists, Smithtown - Ob-Gyn Doctors, Physicians & Surgeons (Obstetrics & Gynecology)
- Off Track Betting, Smithtown
- Office Supplies, Equipment & Office Furniture Stores, Smithtown
- Oil & Petroleum Products, Smithtown - Merchant, Wholesalers, etc
- Oil Burners Services, Smithtown
- Oncologists, Smithtown - Cancer Doctors, Physicians & Surgeons (Oncology)
- Ophthalmologists, Smithtown - Eye Doctors, Physicians & Surgeons (Ophthalmology)
- Optical Stores - Eyeglasses, Lenses & Sunglasses, Smithtown
- Opticians, Smithtown
- Optometrists, Smithtown - Eye Doctors, Physicians & Surgeons (Optometry)
- Oral Surgeon, Smithtown - Dentists - Oral & Maxillofacial Surgeons
- Oriental Restaurants, Smithtown
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- Ornamental Greenhouses, Smithtown
- Ornamental Nursery, Smithtown
- Ornamental Nursery Services, Smithtown
- Orthodontist, Smithtown - Teeth Straightening, Invisalign & Dental Braces
- Orthopedic & Prosthetic Appliances & Shoes, Smithtown - Orthopedic Shoes
- Orthopedists, Smithtown - Orthopedic Surgery Doctors, Physicians & Surgeons
- Osteoporosis Doctors, Smithtown - Physicians & Surgeons
- Other Building Materials & Supplies, Smithtown
- Other Specialties, Smithtown - Doctors, Physicians & Surgeons
- Outdoor & Lawn & Garden Equipment Repair & Service, Smithtown
- Outdoor Equipment & Accessories, Smithtown
- Ownership Organizations, Smithtown
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