- Oskar syndrome
- Oskar von Miller
- OSL Dating
- Oslo
- Oslo Accords
- Oslo Nye Teater
- Oslo University Observatory
- Osmaniya Script
- Osman Muhammad Hashim
- Osman's Paris planning
- osmiophilia
- osmium
- osmium dihydroxylation
- osmium tetroxide
- osmometer
- osmophile
- osmoreceptor
- osmoregulation
- osmosis
- osmotaxis
- osmotic coefficient
- osmotic concentration
- osmotic diuretics
- osmotic fluid sampling
- osmotic hyperoregulation