A highly efficient manganese-promoted oxidative cyclization of unsaturated oximes to afford the corresponding 4,5-dihydroisoxazoline alcohols was developed. A very low loading (generally 0.1–0.2 mol-%) of Mn(acac)3 (acac = acetylacetonate) promoted the oxidative cyclization through the direct incorporation of molecular oxygen present in air (open flask) at room temperature.