A list of diseases in the English wikipedia.

O-Ob

* O Doherty syndrome
* O Donnell Pappas syndrome
* Obesity
* Obesophobia
* Obsessive-compulsive disorder
* Obstructive asymmetric septal hypertrophy
* Obstructive sleep apnea

Oc

Occ-Och

* Occipital horn syndrome
* Occlusive Infantile ateriopathy
* Occult spinal dysraphism
* Occupational asthma
* Occupational asthma - Chemicals
* Occupational asthma - Metals
* Occupational asthma - Plants
* Occupational asthma - Wood dust
* Occupational asthma - Drugs
* OCD
* Ochoa syndrome
* Ochronosis, hereditary
* Ochronosis
* Ockelbo disease

Ocu

Ocul

Ocula

* Ocular Albinism
* Ocular coloboma-imperforate anus
* Ocular convergence spasm
* Ocular Histoplasmosis
* Ocular melanoma
* Ocular motility disorders
* Ocular toxoplasmosis

Oculo

* Oculo-auriculo-vertebral spectrum
* Oculo cerebral dysplasia
* Oculo cerebro acral syndrome
* Oculo cerebro osseous syndrome
* Oculo dento digital dysplasia
* Oculo digital syndrome
* Oculo facio cardio dental syndrome
* Oculo skeletal renal syndrome
* Oculo tricho anal syndrome
* Oculo tricho dysplasia

Oculoa-Oculog

* Oculoauriculofrontonasal syndrome
* Oculoauriculovertebral dysplasia
* Oculocerebral hypopigmentation syndrome Cross type
* Oculocerebral hypopigmentation syndrome type Preus
* Oculocerebral syndrome with hypopigmentation
* Oculocerebrocutaneous syndrome
* Oculocerebrorenal syndrome
* Oculocutaneous albinism immunodeficiency
* Oculocutaneous albinism type 1
* Oculocutaneous albinism type 2
* Oculocutaneous albinism type 3
* Oculocutaneous albinism, tyrosinase negative
* Oculocutaneous albinism, tyrosinase positive
* Oculocutaneous tyrosinemia
* Oculodental syndrome Rutherfurd syndrome
* Oculodentodigital dysplasia dominant
* Oculodentodigital syndrome
* Oculo-dento-digital syndrome
* Oculodentoosseous dysplasia dominant
* Oculodentoosseous dysplasia recessive
* Oculodigitoesophagoduodenal syndrome
* Oculo-gastrointestinal muscular dystrophy

Oculom-Oculor

* Oculomaxillofacial dysostosis
* Oculomaxillofacial dysplasia with oblique facial clefts
* Oculomelic amyoplasia
* Oculopalatoskeletal syndrome
* Oculopharyngeal muscular dystrophy
* Oculorenocerebellar syndrome

Od-Ok

* Odonto onycho dysplasia with alopecia
* Odontoma
* Odontomicronychial dysplasia
* Odontoonychodermal dysplasia
* Odontophobia
* Odontotrichomelic hypohidrotic dysplasia
* Odynophobia
* OFD syndrome type 8
* OFD syndrome type Figuera
* Ogilvie's syndrome
* Ohaha syndrome
* Ohdo Madokoro Sonoda syndrome
* Oikophobia
* Okamuto Satomura syndrome

Ol

* Olfactophobia
* Oligodactyly
* Oligodactyly tetramelia postaxial
* Oligomeganephronic renal hypoplasia
* Oligomeganephrony
* Oligophernia
* Oliver McFarlane syndrome
* Oliver syndrome
* Olivopontocerebellar atrophy deafness
* Olivopontocerebellar atrophy type 1
* Olivopontocerebellar atrophy type 2
* Olivopontocerebellar atrophy type 3
* Olivopontocerebellar atrophy
* Ollier disease
* Olmsted syndrome

Om-On

* Ombrophobia
* Omenn syndrome
* Omodysplasia type 1
* Omphalocele cleft palate syndrome lethal
* Omphalocele exstrophy imperforate anus
* Omphalomesenteric cyst
* Omsk hemorrhagic fever
* Onat syndrome
* Onchocerciasis
* Oncocytoma
* Ondine's curse
* Oneirophobia
* Onychonychia hypoplastic distal phalanges
* Onychotrichodysplasia and neutropenia

Op

Oph

* Ophthalmic icthyosis
* Ophthalmo acromelic syndrome
* Ophthalmomandibulomelic dysplasia
* Ophthalmophobia
* Ophthalmoplegia ataxia hypoacusis
* Ophthalmoplegia mental retardation lingua scrotalis
* Ophthalmoplegia myalgia tubular aggregates
* Opitz Mollica Sorge syndrome
* Opitz Reynolds Fitzgerald syndrome
* Opitz syndrome
* Opportunistic infections
* Oppositional defiant disorder
* Opsismodysplasia

Opt

* Opthalmoplegia progressive external scoliosis
* Optic atrophy opthalmoplegia ptosis deafness myopia
* Optic atrophy polyneuropathy deafness
* Optic atrophy, autosomal dominant
* Optic atrophy, idiopathic, autosomal recessive
* Optic atrophy
* Optic disc drusen
* Optic nerve coloboma with renal disease
* Optic nerve disorder
* Optic nerve hypoplasia, familial bilateral
* Optic neuritis
* Optic pathway glioma
* Opticoacoustic nerve atrophy dementia

Or

Ora-Orn

* Oral facial digital syndrome type 3
* Oral facial digital syndrome type 4
* Oral facial digital syndrome
* Oral facial dyskinesia
* Oral leukoplakia
* Oral lichen planus
* Oral lichenoid lesions
* Oral squamous cell carcinoma
* Oral submucous fibrosis
* Oral-facial cleft
* Oral-facial-digital syndrome, type IV
* Oral-facial-digital syndrome
* Oral-pharyngeal disorders
* Organic brain syndrome
* Organic mood syndrome
* Organic personality syndrome
* Ornithine aminotransferase deficiency
* Ornithine carbamoyl phosphate deficiency
* Ornithine transcarbamylase deficiency, hyperammonemia due to
* Ornithinemia
* Ornithosis

Oro-Ort

* Oro acral syndrome
* Orofaciodigital syndrome Gabrielli type
* Orofaciodigital syndrome Shashi type
* Orofaciodigital syndrome Thurston type
* Orofaciodigital syndrome type 2
* Orofaciodigital syndrome type1
* Orotic aciduria hereditary
* Orotic aciduria purines-pyrimidines
* Orotidylic decarboxylase deficiency
* Orstavik Lindemann Solberg syndrome
* Orthostatic intolerance

Os

Ose-Oss

* Osebold Remondini syndrome
* Osgood-Schlatter disease
* Oslam syndrome
* Osmed Syndrome
* Ossicular Malformations, familial

Ost

Oste

Ostei

* Osteitis deformans

Osteo

: bone

Osteoa-Osteom

* Osteoarthritis
* Osteoarthropathy of fingers familial
* Osteochondritis deformans juvenile
* Osteochondritis dissecans
* Osteochondritis
* Osteochondrodysplasia thrombocytopenia hydrocephalus
* Osteochondroma
* Osteocraniostenosis
* Osteodysplasia familial Anderson type
* Osteodysplastic dwarfism Corsello type
* Osteoectasia familial
* Osteogenesis Imperfecta
* Osteogenesis imperfecta congenita microcephaly and cataracts
* Osteogenesis imperfecta congenital joint contractures
* Osteogenesis imperfecta retinopathy
* Osteogenic sarcoma
* Osteoglophonic dwarfism
* Osteolysis hereditary multicentric
* Osteolysis syndrome recessive
* Osteomalacia
* osteomyelitis

Osteon-Osteos

* Osteonecrosis
* Osteopathia condensans disseminata with osteopoikilosis
* Osteopathia striata cranial sclerosis
* Osteopathia striata pigmentary dermopathy white forelock
* Osteopetrosis autosomal dominant type 1
* Osteopetrosis lethal
* Osteopetrosis renal tubular acidosis
* Osteopetrosis, (generic term)
* Osteopetrosis, malignant
* Osteopetrosis, mild autosomal recessive form
* Osteopoikilosis
* Osteoporosis macrocephaly mental retardation blindness
* Osteoporosis oculocutaneous hypopigmentation syndrome
* Osteoporosis pseudoglioma syndrome
* osteoporosis
* Osteosarcoma limb anomalies erythroid macrocytosis
* Osteosclerose type Stanescu
* Osteosclerosis abnormalities of nervous system and meninges
* Osteosclerosis autosomal dominant Worth type
* Osteosclerosis

Oster

* Ostertag type amyloidosis

Ot-Ox

* Ota Appaura syndrome
* Ota Kawamura Ito syndrome
* Oto palato digital syndrome type I and II
* Otodental dysplasia
* Otofaciocervical syndrome
* Otoonychoperoneal syndrome
* Oto-Palatal-digital syndrome
* Otopalatodigital syndrome type 2
* Otosclerosis, familial
* otosclerosis
* Otospondylomegaepiphyseal dysplasia
* Ouvrier Billson syndrome
* Ovarian cancer
* Ovarian carcinosarcoma
* Ovarian dwarfism as part of Turner Syndrome
* Ovarian dwarfism
* Ovarian insufficiency due to FSH resistance
* ovarian remnant syndrome
* Overfolded helix
* Overgrowth radial ray defect arthrogryposis
* Overgrowth syndrome type Fryer
* Overhydrated hereditary stomatocytosis
* Overwhelming post-splenectomy infection (OPSI)
* Oxalosis