- O'Nyong Nyong fever 066.3
- Oasthouse urine disease 270.2
- Obermeyer's relapsing fever (European) 087.0
- Obesity (constitutional) (exogenous) (familial) (nutritional) (simple) 278.00
- Oblique - see also condition
- Obliquity, pelvis 738.6
- Obliteration
- Observation (for) V71.9
- Obsession, obsessional 300.3
- Obsessive-compulsive 300.3
- Obstetrical trauma NEC (complicating delivery) 665.9
- Obstipation (see also Constipation) 564.00
- Obstruction, obstructed, obstructive
- Obturator - see condition
- Occlusal
- Occlusio pupillae 364.74
- Occlusion
- Occupational
- Ochlophobia 300.29
- Ochronosis (alkaptonuric) (congenital) (endogenous) 270.2
- Ocular muscle - see also condition
- Oculoauriculovertebral dysplasia 756.0