- Onychogryposis 703.8
- Onycholysis 703.8
- Onychomadesis 703.8
- Onychomalacia 703.8
- Onychomycosis 110.1
- Onychophagy 307.9
- Onychoptosis 703.8
- Onychorrhexis 703.8
- Onychoschizia 703.8
- Onychotrophia (see also Atrophy, nail) 703.8
- Onyxis (finger) (toe) 703.0
- Onyxitis (with lymphangitis) 681.9
- Oocyte (egg) (ovum)
- Oophoritis (cystic) (infectional) (interstitial) (see also Salpingo-oophoritis) 614.2
- Opacity, opacities
- Opalescent dentin (hereditary) 520.5
- Open, opening
- Operation
- Operational fatigue 300.89
- Operative - see condition
- Operculitis (chronic) 523.40
- Operculum, retina 361.32