- Obstetricians & Gynecologists, Altoona - Ob-Gyn Doctors, Physicians & Surgeons (Obstetrics & Gynecology)
- Occupational Health Care Professionals, Altoona
- Occupational Therapy, Altoona - Occupational Health Specialists
- Office Supplies, Equipment & Office Furniture Stores, Altoona
- Oil & Petroleum Products, Altoona - Merchant, Wholesalers, etc
- Oil Change & Lubrication, Altoona - Auto Oil & Quick Lube
- Oil Extraction Downhole, Altoona
- Oil Industry & Oil Field Equipment & Services, Altoona - Oil Companies
- Oncologists, Altoona - Cancer Doctors, Physicians & Surgeons (Oncology)
- Online Advertising, Altoona - Internet Advertising
- Ophthalmologists, Altoona - Eye Doctors, Physicians & Surgeons (Ophthalmology)
- Optical Goods Repair & Service, Altoona - Camera Repair
- Optical Stores - Eyeglasses, Lenses & Sunglasses, Altoona
- Opticians, Altoona
- Optometrists, Altoona - Eye Doctors, Physicians & Surgeons (Optometry)
- Optometry Clinics, Altoona - Eye Clinics
- Oral Surgeon, Altoona - Dentists - Oral & Maxillofacial Surgeons
- Orchestras, Symphonies & Bands, Altoona
- Organizing Services & Products – Household & Business, Altoona
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- Ornamental Greenhouses, Altoona
- Ornamental Nursery, Altoona
- Ornamental Nursery Products, Altoona
- Ornamental Nursery Services, Altoona
- Orthodontist, Altoona - Teeth Straightening, Invisalign & Dental Braces
- Orthodox Churches, Altoona
- Orthopedic & Prosthetic Appliances & Shoes, Altoona - Orthopedic Shoes
- Orthopedists, Altoona - Orthopedic Surgery Doctors, Physicians & Surgeons
- Orthotics and Prosthetics, Altoona
- Other Attorneys & Lawyers, Altoona
- Other Attorneys & Lawyers, Altoona
- Other Building Materials & Supplies, Altoona
- Other Lawyers & Attorneys, Altoona
- Other Lodging, Altoona
- Outdoor & Lawn & Garden Equipment Repair & Service, Altoona
- Outdoor Equipment & Accessories, Altoona
- Outdoor Furniture & Patio Furniture, Altoona - Garden Furniture Stores
- Outpatient Services, Altoona - Outpatient Care
- Oxygen & Respiratory Equipment & Supplies, Altoona - Oxygen Concentrator, Tank, Machine, etc
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