This list includes 31 Diseases that start with O that start with O, from “Obesity” to “Overactive Bladder”. These entries cover common, chronic, and infectious conditions useful for quick clinical reference and study.
Diseases that start with O are medical conditions whose established names begin with the letter O. For example, “Obesity” has become a leading global public health issue since the late 20th century.
Below you’ll find the table with the columns: “Disease”, “Symptoms”, “Causes / risk factors”, “Treatments”.
Disease: A concise name for the condition so you can quickly find the entry you need.
Symptoms: Key signs and symptoms (2–4 items) listed to help you quickly recognize typical clinical presentations in practice settings.
Causes / risk factors: Primary causes and top risk factors summarized so you understand likely origins and patient risks.
Treatments: First-line and common management options are noted, plus when you should consider specialist referral or further care.
Diseases that start with O
| Name | ICD-10 code | Main symptoms | Main causes / risk factors |
|---|---|---|---|
| Obesity | E66.9 | Excess body fat, high BMI, increased risk of other diseases | Caloric imbalance, genetic factors, environment, certain medications |
| Obsessive-Compulsive Disorder | F42 | Unwanted thoughts (obsessions), repetitive behaviors (compulsions), distress | Genetic and neurological factors, environmental stressors |
| Ocular Melanoma | C69.4 | Blurry vision, dark spot on iris, change in pupil shape, floaters | Unknown; UV light exposure, light eye color, certain moles are risk factors |
| Ocular Rosacea | L71.8 | Dry eyes, burning or stinging, blurry vision, red eyelids | Unknown; a component of the skin condition rosacea |
| Odontoid Fracture | S12.1 | Severe neck pain, stiffness, neurological symptoms if spinal cord is involved | Trauma to the head or neck, such as from a fall or car accident |
| Oligohydramnios | O41.0 | Uterus smaller than expected, fetal movement may be reduced | Leaking amniotic fluid, placental problems, maternal or fetal health issues |
| Onchocerciasis | B73 | Severe itching, skin rash, vision changes, blindness | Infection with the parasitic worm Onchocerca volvulus, spread by blackfly bites |
| Oppositional Defiant Disorder | F91.3 | Angry/irritable mood, argumentative behavior, vindictiveness | A combination of genetic, environmental, and developmental factors |
| Optic Neuritis | H46 | Pain with eye movement, blurred vision, loss of color vision, vision loss | Autoimmune reaction, often associated with multiple sclerosis (MS) |
| Oral Cancer | C06.9 | A sore that doesn’t heal, a lump in the mouth, pain, difficulty swallowing | Tobacco use, heavy alcohol consumption, HPV infection |
| Oral Lichen Planus | L43.8 | White lacy patches on gums/tongue, red swollen tissues, mouth pain | Autoimmune response; triggers can include stress or certain medications |
| Oral Thrush | B37.0 | White patches on tongue/cheeks, soreness, loss of taste | Overgrowth of the fungus Candida albicans; weakened immune system, antibiotic use |
| Orchitis | N45 | Testicular pain and swelling, fever, nausea and vomiting | Bacterial or viral infection (e.g., mumps, STIs) |
| Ornithine Transcarbamylase Deficiency | E72.21 | Lethargy, vomiting, seizures, intellectual disability | Genetic mutation affecting the urea cycle, leading to high ammonia levels |
| Orthostatic Hypotension | I95.1 | Dizziness upon standing, lightheadedness, fainting, blurred vision | Drop in blood pressure after standing up; dehydration, medication side effects |
| Osgood-Schlatter Disease | M92.5 | Knee pain, swelling below the kneecap, tenderness over the shinbone | Stress on the growth plate from repetitive quadriceps contraction |
| Osteoarthritis | M19.90 | Joint pain, stiffness, decreased flexibility, bone spurs | Wear and tear on joint cartilage; age, obesity, prior injury |
| Osteochondritis Dissecans | M93.2 | Joint pain, swelling, catching or locking of the joint | Reduced blood flow to bone, causing a piece to separate |
| Osteogenesis Imperfecta | Q78.0 | Bones that break easily, bone deformities, blue sclerae (whites of eyes) | Genetic mutations affecting collagen production |
| Osteomalacia | M83.9 | Dull, aching bone pain, muscle weakness, difficulty walking | Severe vitamin D deficiency, problems with vitamin D metabolism |
| Osteomyelitis | M86.9 | Bone pain, fever, swelling and redness over the area, fatigue | Bacterial infection in the bone, from injury, surgery, or the bloodstream |
| Osteonecrosis | M87 | Joint pain that may become severe, limited range of motion | Loss of blood supply to bone; long-term steroid use, excessive alcohol use |
| Osteopetrosis | Q78.2 | Frequent fractures, bone pain, vision/hearing problems, anemia | Genetic mutations that impair bone resorption by osteoclasts |
| Osteoporosis | M81.0 | Often no symptoms until a fracture, loss of height, stooped posture | Low bone mass and structural deterioration; aging, female sex, low calcium |
| Osteosarcoma | C40.9 | Bone pain (worse at night), swelling near a bone, a broken bone | Unknown; associated with rapid bone growth, genetic factors, prior radiation |
| Otitis Externa | H60.9 | Ear pain, itching in ear canal, redness, discharge from ear | Infection of the outer ear canal; excess moisture (swimmer’s ear), scratches |
| Otitis Media | H66.9 | Ear pain, fluid drainage, hearing difficulty, fever | Bacterial or viral infection of the middle ear; common in young children |
| Otosclerosis | H80.9 | Gradual hearing loss, ringing in the ear (tinnitus), dizziness | Abnormal bone remodeling in the middle ear, often has a genetic component |
| Ovarian Cancer | C56 | Bloating, pelvic or abdominal pain, feeling full quickly, urinary urgency | Unknown; genetic mutations (BRCA), age, family history are risk factors |
| Ovarian Cysts | N83.2 | Pelvic pain, bloating, fullness in the abdomen | Fluid-filled sacs on an ovary; often form during the menstrual cycle |
| Overactive Bladder | N32.81 | Sudden urge to urinate, frequent urination, nocturia | Involuntary bladder muscle contractions; nerve damage, certain medications |
Descriptions
Obesity
A complex disease involving an excessive amount of body fat. Management includes lifestyle changes like diet and exercise, and may involve medication or surgery. A primary care provider can coordinate care.
Obsessive-Compulsive Disorder
A mental health disorder characterized by a cycle of obsessions and compulsions. Treatment involves psychotherapy (CBT) and medication (SSRIs). Managed by a psychiatrist or therapist.
Ocular Melanoma
A rare but serious cancer of the eye. Early detection is key. Treatment may include radiation, laser therapy, or surgery to remove the eye. Requires immediate referral to an ophthalmologist.
Ocular Rosacea
An inflammatory condition affecting the eyes, common in people with skin rosacea. Managed with artificial tears, eyelid hygiene, and sometimes oral antibiotics. An ophthalmologist should be consulted.
Odontoid Fracture
A fracture of the dens, a bony peg in the second cervical vertebra (C2). It can be a serious injury requiring immobilization or surgery. Requires immediate emergency care.
Oligohydramnios
A condition in pregnancy characterized by a deficiency of amniotic fluid. It requires close monitoring and management by an obstetrician, as it can affect fetal development and delivery.
Onchocerciasis
Also known as “river blindness,” this parasitic disease affects the skin and eyes. It is treated with the anti-parasitic drug ivermectin. Management involves infectious disease specialists.
Oppositional Defiant Disorder
A childhood behavioral disorder characterized by a persistent pattern of defiance and hostility. Treatment includes family and individual therapy. Managed by a child psychiatrist or psychologist.
Optic Neuritis
Inflammation of the optic nerve, which sends visual information from the eye to the brain. Treatment may include steroids. Requires urgent evaluation by a neurologist and ophthalmologist.
Oral Cancer
Cancer that develops in any part of the mouth. Early detection is critical for successful treatment, which may include surgery, radiation, and chemotherapy. Managed by an oncologist and ENT surgeon.
Oral Lichen Planus
A chronic inflammatory condition affecting mucous membranes inside your mouth. Management focuses on symptom relief with corticosteroids. Often managed by a dentist, dermatologist, or oral surgeon.
Oral Thrush
A yeast infection that develops on the mucous membranes of the mouth. It is treated with antifungal medications. A primary care provider or dentist can diagnose and treat it.
Orchitis
Inflammation of one or both testicles, often caused by an infection. Treatment includes antibiotics for bacterial causes and supportive care. Requires medical evaluation by a urologist or primary care physician.
Ornithine Transcarbamylase Deficiency
A rare genetic disorder that causes ammonia to accumulate in the blood. Management involves a strict low-protein diet and medications. Requires lifelong care from a metabolic specialist.
Orthostatic Hypotension
A form of low blood pressure that happens when you stand up from sitting or lying down. Treatment involves addressing the underlying cause and lifestyle changes. Managed by a primary care doctor or cardiologist.
Osgood-Schlatter Disease
A common cause of knee pain in growing adolescents, involving inflammation where the patellar tendon attaches to the shinbone. It resolves with time and is managed with rest, ice, and pain relievers.
Osteoarthritis
The most common form of arthritis, where protective cartilage on the ends of bones wears down over time. Management includes exercise, weight control, and pain relievers. May require orthopedic surgery.
Osteochondritis Dissecans
A joint condition where a small segment of bone and cartilage begins to separate from its surrounding region due to a lack of blood supply. Treatment may involve rest or surgery. Managed by an orthopedic surgeon.
Osteogenesis Imperfecta
Also known as “brittle bone disease,” this is a group of genetic disorders that primarily affect the bones. Management focuses on fracture care and maximizing mobility. Requires a multidisciplinary team.
Osteomalacia
A disease that weakens bones and can cause them to break more easily, often called “soft bones.” Treatment involves replenishing vitamin D and calcium levels. Managed by a primary care doctor or endocrinologist.
Osteomyelitis
A serious infection of the bone. Treatment requires a long course of antibiotics and may involve surgery to remove infected bone tissue. Managed by an infectious disease specialist and an orthopedic surgeon.
Osteonecrosis
Also known as avascular necrosis, this is the death of bone tissue due to a lack of blood supply. Can lead to tiny breaks in the bone and the bone’s eventual collapse. May require joint replacement.
Osteopetrosis
A rare genetic disease that makes bones abnormally dense and prone to fracture. It can cause a range of other problems due to bone overgrowth. Treatment is supportive and managed by specialists.
Osteoporosis
A condition where bones become weak and brittle, increasing fracture risk. Managed with medication, calcium/vitamin D supplements, and exercise. A primary care physician or endocrinologist can manage it.
Osteosarcoma
The most common type of bone cancer, typically developing in teenagers during growth spurts. Treatment involves chemotherapy, surgery, and sometimes radiation. Requires an oncology team.
Otitis Externa
An infection of the outer ear canal, often called “swimmer’s ear.” It is typically caused by bacteria. Treatment involves medicated ear drops. A primary care provider can treat most cases.
Otitis Media
A middle ear infection, very common in children and often follows a cold. Many resolve on their own, but antibiotics may be prescribed. Diagnosed and treated by a pediatrician or family doctor.
Otosclerosis
A condition where abnormal bone growth in the middle ear causes hearing loss. Treatment options include hearing aids or a surgical procedure called a stapedectomy. Managed by an ENT specialist.
Ovarian Cancer
A cancer that begins in the ovaries. It often has vague symptoms and is detected late. Treatment includes surgery and chemotherapy. Requires management by a gynecologic oncologist.
Ovarian Cysts
Commonly develop during childbearing years. Most are harmless and disappear without treatment, but some can cause pain or other complications. An OB-GYN can monitor and manage them.
Overactive Bladder
A condition characterized by a sudden, uncontrolled need to urinate. Management includes behavioral therapies, pelvic floor exercises, and medication. A urologist or primary care provider can help.