Background: The population worldwide is rapidly ageing, and demand for intensive care is increasing. People aged 85 years and above, known as the oldest old, are particularly vulnerable to critical illness owing to the physiological effects of ageing. Evidence surrounding admission of the oldest old to the intensive care is limited. Objective: The objective of this study was to systematically and comprehensively review and synthesise the published research investigating factors that influence decisions to admit the oldest old to the intensive care unit. Method: This was a systematic review and narrative synthesis. Following a comprehensive search of CINAHL, Embase, and Medline databases, peer-reviewed primary research articles examining factors associated with admission or refusal to admit the oldest old to intensive care were selected. Data were extracted into tables and narratively synthesised. Results: Six studies met the inclusion criteria. Three studies identified factors associated with admission such as greater premorbid self-sufficiency, patient preferences, alignment between patient and physicians’ goals of treatment, age less than 85 years, and absence of cancer, or previous intensive care admission. Factors associated with refusal to admit were identified in all six studies and included limited or no bed availability, level of ICU physician experience, patients being deemed too ill or too well to benefit, and older age. Conclusions: Published research investigating decision-making about admission or refusal to admit the oldest old to the intensive care unit is scant. The ageing population and increasing demand for intensive care unit resources has amplified the need for greater understanding of factors that influence decisions to admit or refuse admission of the oldest old to the intensive care unit. Such knowledge may inform guidelines regarding complex practice decisions about admission of the oldest old to an intensive care unit. Such guidelines would ensure the specialty needs of this population are considered and would reduce admission decisions that might disadvantage older people.