oesophagus – part of the gut between the throat and the stomach.

On-demand dosing – a term for taking medicines only at times linked to a relevant event.

On-demand dosing for PrEP is also called “event based dosing” (EBD).

For example, on-demand dosing for PrEP (to prevent HIV transmission) involves taking PrEP before sex and then for several days after sex. EBD for PrEP is only appropriate in some situations and for some people – otherwise daily dosing is recommended,

open label – this means that a patient in a trial knows which treatment they are taking.

opiate – a drug that comes from the opium poppy. Opiates include heroin, codeine and morphine.

opportunistic infection (OI) – an infection that occurs after your immune system has been damaged by HIV.

OI chapter of the training manual.

organogenesis – the process of development of baby’s main organs. This period is the first 12 weeks in the womb.

outreach – a type of service which goes out to find and engage people.