In this work we report on a study of the mechanism of O-electron stimulated desorption (ESD) from hydrogenated and hydrogen-free polycrystalline diamond films exposed to thermally activated oxygen for incident electron energies in the 4-22 eV range. Two types of experiments were carried out in order to assess the nature of the ESD processes: (i) total O-and H- yields as a function of incident electron energy and (ii) kinetic-energy distribution (KED) of O- desorbed from the hydrogen-free diamond surface. The discussed ESD mechanism is referred to the information obtained from x-ray photoelectron spectroscopy, near-edge x-ray absorption fine structure, and core level H+ photodesorption measurements which reveal formation of Cdouble bondO and C-O-C bonds on the hydrogen-free diamond surface, and Cdouble bondO and C-O-H bonds on the hydrogenated one. Based on the maximum kinetic-energy value of O- and the ESD threshold measured for hydrogen-free surface, all low-energy (5-10 eV) O- ions are attributed to desorption by the dissociative electron attachment (DEA) to C-O-C, while DEA to Cdouble bondO occurs at the incident electron energy higher than similar to10 eV. O- ESD from the hydrogenated diamond surface exposed to thermally activated oxygen is a more complicated process. Its threshold is substantially higher than for hydrogen-free diamond, and the line shape of the ESD yield curve is very similar to that of chemisorbed CO molecules. Several reaction pathways leading to production of O- by DEA are discussed for this sample. At incident electron energies higher than similar to15 eV, O- ESD proceeds also via dipolar dissociation processes for both hydrogenated and hydrogen-free diamond surfaces.