The peri-, chemo-, stereo-, and regioselectivity of the addition of the transition-metal oxides OsO4 and LReO3 (L = O-, H3PN, Me, Cp) to ketene were systematically investigated using density-functional methods. While metal-oxide additions to ethylene have recently been reported to follow a 3+2 mechanism only, the calculations reveal a strong influence of the metal on the periselectivity of the ketene addition:  OsO4 again prefers a 3+2 pathway across the CC moiety whereas, for the rhenium oxides LReO3, the 2+2 barriers are lowest. Furthermore, a divergent chemoselectivity arising from the ligand L was found:  ReO4 - and (H3PN)ReO3 add across the CO bond while MeReO3 and CpReO3 favor the addition across the CC moiety. The calculated energy profile for the MeReO3 additions differs from the CpReO3 energy profile by up to 45 kcal/mol due to the stereoelectronic flexibility of the Cp ligand adopting η5, η3, and η1 bonding modes. The selectivity of the cycloadditions was rationalized by the analysis of donor−acceptor interactions in the transition states. In contrast, metal-oxide additions to diphenylketene probably follow a different mechanism:  We give theoretical evidence for a zwitterionic intermediate that is formed by nucleophilic attack at the carbonyl moiety and undergoes a subsequent cyclization yielding the thermodynamically favored product. This two-step pathway is in agreement with the results of recent experimental work.