- O... to Oculoauriculovertebral...
- Oculogyric... to Ohara...
- Oidiomycosis... to Oliguria...
- Ollier... to Onychogryphosis...
- Onychogryposis... to Operculum...
- Ophiasis... to Oriental...
- Orientation... to Osteo...
- Osteo... to Osteofibrosarcoma...
- Osteogenesis... to Osteopsathyrosis...
- Osteoradionecrosis... to Outburst...
- Outcome... to Overhanging...
- Overload... to Ozena...