Switchable polymerization affords a unique opportunity to regulate polymer sequence and structure in a one-pot process from mixtures of monomers. Here we report a strategy that uses O2 as an external stimulus to switch the polymerization mechanism from the organometallic mediated radical polymerization (OMRP) of vinyl monomers mediated by (Salen)Co III-R [Salen=N,N'-bis(3,5-di-tertbutylsalicylidene)-1,2-cyclohexanediamine; R = alkyl] to the ringopening copolymerization (ROCOP) of CO2/epoxides. A critical issue is the unprecedented mono oxygen insertion into the Co-C bond, as demonstrated by experimental results and rationalized by DFT calculations, leading to the formation of (Salen)Co III-OR as an active species to initiate ROCOP. Diblock poly(vinyl acetate)-bpolycarbonate could be obtained from ROCOP of CO2/epoxides with preactivation of (Salen)Co end-capped poly(vinyl acetate). Furthermore, a poly(vinyl acetate)-b-poly(methyl acrylate)-bpolycarbonate triblock copolymer has been successfully synthesized via a (Salen)cobalt-mediated sequential polymerization with an O2triggered switch in a one-pot process.