Oesophageal atresia is a rare birth defect that affects a baby's oesophagus (the tube through which food passes from the mouth to the stomach).
Read about orf, a viral skin disease that can be spread to humans by handling infected sheep and goats.
Find out about osteophytes, which are bony lumps (bone spurs) that grow on the bones of the spine or around the joints.
Find out about otosclerosis, including the symptoms, causes and treatments.
Find out about ovulation pain including how to treat it, it's affect on your fertility, and when you need to see a doctor.
Read about obesity and how it can seriously affect your health. Ways to lose weight safely include eating a healthy, reduced-calorie diet and exercising regularly.
Find out about the symptoms of obsessive compulsive disorder (OCD), what causes it, where to get help, and what the main treatments are.
Read about occupational therapy, which provides support to people whose health prevents them doing the activities that matter to them.
Understand more about what oesophageal cancer is.
Read about orthodontics, a type of dentistry that aims to improve the appearance, position and function of crooked, protruding or crowded teeth.
Read about osteoarthritis, a condition that causes the joints to become painful and stiff. It's the most common type of arthritis in the UK.
Read about osteomyelitis, which is a bone infection that’s usually caused by bacteria. Find out when to see a GP, what will happen at your appointment and how osteomyelitis is treated.
Osteopathy is a way of detecting, treating and preventing health problems by moving, stretching and massaging a person's muscles and joints.
Find out about osteoporosis, a health condition that weakens bones, making them fragile and more likely to break. There is also information on osteopenia, the stage of bone loss before osteoporosis, and how to stop it becoming osteoporosis.
Read more about ovarian cancer, which is a cancer that begins in the ovaries - 2 small organs that store women's eggs
Read about an ovarian cyst, a fluid-filled sac that develops on a woman's ovary. They're very common and do not usually cause any symptoms
Find out about overactive thyroid (hyperthyroidism), including what the symptoms are, when to see your GP, how it's treated and why it happens.
Read more about oral thrush, which is usually harmless. It's common in babies and older people with dentures. It can be easily treated with medicines prescribed by a GP.