- Office Equipment & Supplies Manufacturers, Fairfield
- Office Machinery, Fairfield
- Office Supplies, Equipment & Office Furniture Stores, Fairfield
- Offset Printers Supplies, Fairfield
- Oil & Gas Companies, Fairfield
- Ophthalmic Goods, Fairfield
- Ophthalmic Instruments & Machinery, Fairfield
- Optical Goods Manufacturers, Fairfield
- Optical Goods Repair & Service, Fairfield - Camera Repair
- Optical Stores - Eyeglasses, Lenses & Sunglasses, Fairfield
- Ornamental Greenhouses, Fairfield
- Ornamental Nursery, Fairfield
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- Ornamental Nursery Services, Fairfield
- Orthopedic & Prosthetic Appliances & Shoes, Fairfield - Orthopedic Shoes
- Osteoporosis Doctors, Fairfield - Physicians & Surgeons
- Other Attorneys & Lawyers, Fairfield
- Other Attorneys & Lawyers, Fairfield
- Other Building Materials & Supplies, Fairfield
- Other Lawyers & Attorneys, Fairfield
- Outdoor & Lawn & Garden Equipment Repair & Service, Fairfield
- Outdoor Advertising & Billboard, Fairfield
- Outdoor Equipment & Accessories, Fairfield
- Outerwear & Accessories Manufacturers, Fairfield
- Oxygen & Respiratory Equipment & Supplies, Fairfield - Oxygen Concentrator, Tank, Machine, etc
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