A novel oxygen-atom-transfer process enables the catalytic [2 + 2 + 1] synthesis of bicyclic furans from alpha,omega-diynes with DMSO. [CpRu(AN)(3)]PF6 catalyzed the transfer oxygenative cyclization of diynes with aryl terminal groups, while those of diynes with alkyl terminal groups were effectively promoted by the corresponding Cp* complex. A mechanism for bicyclic furan formation via a ruthenacyclopentatriene was proposed on the basis of both experimental and theoretical studies.