Despite the promising role of cyclopentanone as a bioderived fuel, thermodynamic and kinetic data are lacking for low temperature oxidation regimes. In this study, ab initio calculations at the CBS-QB3 level explore the subsequent reactivity that results from O-2-addition to 2- and 3-oxo cyclopentyl radicals, including expected reaction classes such as intra-H migration, HO2-elimination, cyclic ether formation, and beta-scission along with their thermodynamic parameters. Some of the rates are similar to the analogous reactions of cyclopentane, but some other reactions of cyclopen- tanone are very different. The carbonyl group hinders H-migration from the alpha' position but promotes HO2-elimination. Enol peroxy formation from some hydroperoxy alkyl radicals of cyclopentanone is unexpectedly important, and so is HO2-elimination by beta-scission. Our calculations also indicated that at engine relevant conditions the alpha-RO2 prefers to go back to the reactants 2-oxo cyclopentyl radical and O-2. Therefore, the reactions resulting from HO2-addition to 2-oxo cyclopentyl are also provided. The lowest barrier channel identified on the singlet surface corresponds to an unexpected intra OH-migration path concerted with ring opening. This valuable information will advance the construction of improved kinetic models for the oxidation of cylopentanone.