Quantum chemical calculations by using density functional theory at the B3LYP level have been carried out to elucidate the reaction course for the addition of ethylene to [OsO2(CH2)2] (1). The calculations predict that the kinetically most favorable reaction proceeds with an activation barrier of 8.1 kcal mol−1 via [3+2] addition across the OOsCH2 moiety. This reaction is −42.4 kcal mol−1 exothermic. Alternatively, the [3+2] addition to the H2COsCH2 fragment of 1 leads to the most stable addition product 4 (−72.7 kcal mol−1), yet this process has a higher activation barrier (13.0 kcal mol−1). The [3+2] addition to the OOsO fragment yielding 2 is kinetically (27.5 kcal mol−1) and thermodynamically (−7.0 kcal mol−1) the least favorable [3+2] reaction. The formal [2+2] addition to the OsO and OsCH2 double bonds proceeds by initial rearrangement of 1 to the metallaoxirane 1 a. The rearrangement 1→1 a and the following [2+2] additions have significantly higher activation barriers (>30 kcal mol−1) than the [3+2] reactions. Another isomer of 1 is the dioxoosmacyclopropane 1 b, which is 56.2 kcal mol−1 lower in energy than 1. The activation barrier for the 1→1 b isomerization is 15.7 kcal mol−1. The calculations predict that there are no energetically favorable addition reactions of ethylene with 1 b. The isomeric form 1 c containing a peroxo group is too high in energy to be relevant for the reaction course. The accuracy of the B3LYP results is corroborated by high level post-HF CCSD(T) calculations for a subset of species.