Osmium is a heavy transition metal noted for its exceptional catalytic activity in oxidation and hydrogenation reactions. Osmium tetroxide (OsO₄) is a well-known reagent for the dihydroxylation of alkenes, producing vicinal diols with high regio- and stereoselectivity. Osmium catalysts can also mediate asymmetric dihydroxylations when combined with chiral ligands (Sharpless-type dihydroxylation). Osmium complexes are utilized in hydrosilylation and hydrogenation of unsaturated substrates, though their toxicity and cost limit large-scale use. Recent developments focus on immobilizing osmium catalysts to improve recyclability and reduce environmental impact. Osmium’s high oxidation states and strong Lewis acidity enable unique activation modes for organic substrates. Despite being less common than ruthenium or rhodium, osmium remains a valuable catalyst for specialized organic transformations requiring high selectivity and efficiency.