Observational studies include case-control and cohort studies.  In an observational study, the researcher observes and collects data on populations of people.  Groups of people with different exposures are compared to see if these exposures are associated with disease.  An observational study differs from a clinical trial in that people in an observational study are not randomized, and instead, choose their own exposures.

Outcome is the result of a process, including outputs, effects and impacts.

Outcomes research studies the "end results" of particular health care practices and interventions.  These results include how long people live (mortality), what illnesses they get, and how well they feel (Quality of Life).  Outcomes research links the care people get to how it affects their lives.  This type of research is one key to developing better care because it is essential in establishing standards of care, and hence, measuring the quality of care.

Out-of-pocket costs are the health care costs that patients have to pay themselves.  Such costs can include co-payments, deductibles, and costs over and above a health plan's limits.  They can also be costs for treatment that an individual's health plan does not cover.

Outpatient Care is medical or surgical care that does not include an overnight hospital stay.

Output is the flow of patients out of a medical facility, such as an emergency department.

Overcrowding is a situation experienced by many emergency departments across the nation where there are too many patients for the number of physicians available. This causes excessive time spent waiting for care, unhappy patients and poor-quality care provided to patients.

Overuse describes a process of care in circumstances where the potential for harm exceeds the potential for benefit. Prescribing an antibiotic for a viral infection like a cold, for which antibiotics are ineffective, constitutes overuse. The potential for harm includes adverse reactions to the antibiotics and increases in antibiotic resistance among bacteria in the community. Overuse can also apply to diagnostic tests and surgical procedures.