- OCCUR
- see Japan Association for the Lesbian and Gay Movement - Oceania
- see Pacific/Oceania - Older people
- Open Society Institute (1993 - )
- Opioid substitution therapy
- see Injecting drug use - Opportunisitc infection
- Orphans and other children made vulnerable by AIDS
- Oxfam (UK) (1942 - )
- Oxfam Hong Kong (1976 - )