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Background: Ojas is described in Ayurveda as the essence of all dhatus and the basis of strength and resistance to disease. Together with tejas and prana it forms a triad of vital principles. The concept is often compared to immunity but its classical scope is broader. Objective: To review the classical description of ojas, its types, functions and disorders, the associated concepts of tejas and prana, and recent attempts to link ojas with immunity, cholesterol-derived molecules and clinical outcomes. Methods: Classical texts were read in translation. PubMed was searched in November 2026 for papers on ojas, immunity and vyadhikshamatva in Ayurveda. Results: Ojas is described as unctuous, cool, stable and clear, located in the heart and pervading the body. Two kinds are recognised: para (central, in limited quantity) and apara (circulating). Its disorders are visramsa, vyapat and kshaya. The classical texts link its state to strength, immunity and survival. A recent hypothesis proposes cholesterol and its products as candidate biomarkers for ojas, and clinical studies describe agni and ojas as outcome concepts in diabetes. Conclusion: Ojas is a multidimensional concept that includes but is not limited to immunity. Operational definitions and biomarker studies are needed before its relationship with immune and metabolic parameters can be established.