| Gene |
Chromosome |
Location |
Syndrome |
Category |
| OAS1 | 12 | q24.13 | Type 1 Diabetes | Diabetes | |
| OAT | 10 | q26.13 | Gyrate atrophy | Cataracts | |
| OBHD | 9 | q21.33 | OBESITY HYPERPHAGIA AND DEVELOPMENTAL DELAY | Autism spectrum | |
| OCA2 | 15 | q12 | Oculocutaneous albinism type 2 | Photophobia - sensitivity to light | |
| OCA2 | 15 | q13.1 | Microcephaly - other | Microcephaly | |
| OCA2 | 15 | q12-13.1 | Angelman`s Syndrome | Autism spectrum | |
| OCA2 | 15 | q21.3 | Prader-Willi | Autism spectrum | |
| OCNDS | 20 | p13 | OKUR-CHUNG NEURODEVELOPMENTAL SYNDROME | Autism spectrum | |
| OFD1 | 23 | p22 | Oral-facial-digital syndrome | Cleft lip / cleft palate | |
| OGDNS | 23 | q28 | OGDEN SYNDROME | Autism spectrum | |
| OGT | 23 | q13 | Alzheimer - late onset | Alzheimer Syndrome | |
| OI15 | 12 | q13.12 | OSTEOGENESIS IMPERFECTA TYPE XV | Autism spectrum | |
| OLR1 | 12 | p12.3-13.2 | Alzheimer - late onset | Alzheimer Syndrome | |
| OPA1-both | 3 | q29 | Behr syndrome | Ataxia spectrum | |
| OPA1-one | 3 | q29 | Optic atrophy type 01 | Ataxia spectrum | |
| OPA3-both | 19 | q13.32 | Costeff syndrome | Cataracts | |
| OPA3-one | 19 | q13.32 | Autosomal dominant optic atrophy and cataract (ADOAC) | Cataracts | |
| OPHN1 | 23 | q12 | MENTAL RETARDATION X-LINKED WITH CEREBELLAR HYPOPLASIA | Autism spectrum | |
| OPN1LW | 23 | q28 | Color vision deficiency | Photophobia - sensitivity to light | |
| OPN1MW | 23 | q28 | Color vision deficiency | Photophobia - sensitivity to light | |
| OPN1SW | 7 | q32.1 | Color vision deficiency | Photophobia - sensitivity to light | |
| OPN4 | 10 | q23.2 | Seasonal affective disorder | Sleep disorders | |
| OPRM1 | 6 | q25.2 | OPIOID RECEPTOR MU-1 | Autism spectrum | |
| OPTN | 10 | p13 | ALS | ALS - amyotrophic lateral sclerosis | |
| ORC1 | 1 | p32 | Meier-Gorlin syndrome | Microcephaly | |
| ORC4 | 2 | q22-23 | Meier-Gorlin syndrome | Microcephaly | |
| ORC6 | 16 | q12 | Meier-Gorlin syndrome | Microcephaly | |
| OTOP1 | 4 | p16.3 | Autism 18 (AUTS18) | Autism spectrum | |
| OTUD7A | 15 | q13.3 | OTU DOMAIN-CONTAINING PROTEIN 7A | Autism spectrum | |
| OTX2 | 14 | q22.3 | Microphthalmia isolated with coloboma 3 (MCOPCB3) | Microphthalmia | |