Reports from the Food and Drug Administration (FDA) and the Joint Commission on Accreditation of Healthcare Organizations (JCAHO) have emphasized the potential for injury to patients caused by failures in oxygen supply systems. This article presents a model of patient risk related to the process of supplying oxygen at a single university hospital. One of the goals of the article is to illustrate how probabilistic risk analysis (PRA) can be used by hospitals to assess and mitigate risk and, therefore, to meet JCAHO requirements. PRA techniques are useful to 1) model the reliability of a complex system and 2) assess the cost-effectiveness of different risk mitigation measures. The authors focus on the risk estimation step, describing in detail their modeling of the oxygen supply system and analysis of the results. For the hospital that the authors study (20,000 admissions yearly), the total expected number of fatalities from oxygen system failure is 44 over a 30-year time horizon. The greatest contribution to the risk (94% of the expected number of fatalities) comes from problems that involve the supply network (e.g., damage to structure and poisoning) as opposed to incidents that occur inside patient rooms. Although the threat to patient safety is not dramatic, health care organizations should be concerned about potential failures of their oxygen system because improving this system could avoid low-probability, high-consequence failures at a low cost.