See Rare Diseases by First Letter (O)
- Occipital Horn Syndrome
- Ochronosis
- Ocular Cicatricial Pemphigoid
- Ocular Melanoma
- Ocular Motility Disorders
- Ocular Toxoplasmosis
- Oculocutaneous Albinism
- Oculodentodigital Dysplasia
- Oculomotor Apraxia Cogan Type
- Oculopharyngeal Muscular Dystrophy
- Oligoastrocytoma
- Oligodendroglioma
- Omenn Syndrome
- Onchocerciasis
- Oncogenic Osteomalacia
- Optic Atrophy 1
- Optic Neuritis
- Optic Neuropathy, Anterior Ischemic
- Optic Pathway Glioma
- Oral Cancer
- Oral Leukoplakia
- Oral Lichen Planus
- Oral Squamous Cell Carcinoma
- Orbital Lymphoma
- Organic Acidemia
- Ornithine Transcarbamylase Deficiency
- Orofaciodigital Syndromes
- Oropharyngeal Cancer, Adult
- Osteochondritis Dissecans
- Osteochondroma
- Osteogenesis Imperfecta
- Osteomalacia
- Osteomyelitis
- Osteopetrosis
- Osteoporosis-pseudoglioma Syndrome
- Osteosarcoma
- Oto-Palatal-digital Syndrome
- Oto-palato-digital Syndrome Type 1
- Ovarian Cancer
- Ovarian Carcinosarcoma
- Ovarian Epithelial Cancer
- Ovarian Germ Cell Tumor
- Ovarian Low Malignant Potential Tumor