Glossary Please select an letter from the A-Z list or search # A B C D E F G H I J K L M N O P Q R S T U V W X Y Z observational study A study design in which patients receive routine clinical care and researchers record the outcome. Observational studies can provide useful information but are considered less reliable than experimental studies such as randomised controlled trials. Some examples of observational studies are cohort studies and case-control studies. obstetric Relating to antenatal care. occult hepatitis B When a person has a negative result on a HBs Ag test, but has detectable hepatitis B DNA in blood. ocular Relating to the eye. od Abbreviation of a Latin term meaning every day. odds Expresses how likely a person is to experience an event, compared to how likely the person is not to experience the event. An odds above 1 means that the event is more likely to happen than not. odds ratio Comparing one group with another, expresses differences in the odds of something happening. An odds ratio above 1 means something is more likely to happen in the group of interest; an odds ratio below 1 means it is less likely to happen. Similar to ‘relative risk’. oedema Accumulation of fluid below the skin or in the cavities of the body. oesophagus The tube leading from the throat to the stomach. OI Opportunistic infection. om Abbreviation of a Latin term meaning every morning. on Abbreviation of a Latin term meaning every night. on treatment analysis Participants in a clinical trial are only included in the final analysis if they complete the full course of treatment they were originally assigned to. See also ‘intent to treat analysis’. open-label A clinical trial where both the researcher and participants know who is taking the experimental treatment. opportunistic infection Specific infections which cause disease in someone with a damaged immune system. oral regimen A combination of drugs which is taken by mouth - i.e. tablets, without any injections.