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MgO-supported osmium dioxo species, described as Os(=O)2 {-Osupport }1 or 2 (the brackets denote O atoms of the MgO surface), formed from Os3 (CO)12 via supported Os(CO)2 , and characterized by spectroscopy, microscopy, and theory, react with ethylene at 298 K to form osmium glycol species or with CO to give osmium mono- and di-carbonyls. Os(=O)2 {-Osupport }1 or 2 is the precursor of a CO oxidation catalyst characterized by a turnover frequency of 4.0×10-3 (molecules of CO)/(Os atom×s) at 473 K; the active species are inferred to be osmium monocarbonyls. The structures and frequencies calculated at the level of density functional theory with the B3LYP functional bolster the experimental results and facilitate structural assignments. The lowest-energy structures have various osmium oxidation and spin states. The results demonstrate not only new chemistry of the supported single-site catalysts but also their complexity and the value of complementary techniques used in concert to unravel the chemistry.