OIS
- Oberoi International School
- Object Oriented Input System
- Objective Interface Systems
- Ocular ischemic syndrome
- Oeiras International School
- Ops in Surgery, The St George's Surgical Society
- Optical IP Switching
- Optical image stabilization
- Orchlon school
- Order of Ikhamanga
- Organised Independents
- Oslo International School
- Oulu International School
- Outbreak intensity score
- Overnight indexed swap
- Oxford International School (Panama)
- Oxygen isotope stage
- Senescence
- Örgryte IS