- Office Supplies, Equipment & Office Furniture Stores, Fairfield
- Oil & Gas Exploration & Development, Fairfield - Oil Field Exploration, Monitoring & Remediation
- Oil Brokers, Fairfield
- Oil Change & Lubrication, Fairfield - Auto Oil & Quick Lube
- Oil Industry & Oil Field Equipment & Services, Fairfield - Oil Companies
- Oncologists, Fairfield - Cancer Doctors, Physicians & Surgeons (Oncology)
- Optical Stores - Eyeglasses, Lenses & Sunglasses, Fairfield
- Optometrists, Fairfield - Eye Doctors, Physicians & Surgeons (Optometry)
- Ornamental Nursery, Fairfield
- Ornamental Nursery Services, Fairfield
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- Orthodontist, Fairfield - Teeth Straightening, Invisalign & Dental Braces
- Other Attorneys & Lawyers, Fairfield
- Other Attorneys & Lawyers, Fairfield
- Other Building Materials & Supplies, Fairfield
- Other Lawyers & Attorneys, Fairfield
- Other Lodging, Fairfield
- Outdoor & Lawn & Garden Equipment Repair & Service, Fairfield
- Outerwear Stores, Fairfield - Jackets, Sunglasses, Hats, Caps, Coats, Gloves & More
- Outpatient Services, Fairfield - Outpatient Care
- Oxygen & Respiratory Equipment & Supplies, Fairfield - Oxygen Concentrator, Tank, Machine, etc
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