In this study, the stopping power for electrons in O-2 and O-3 molecules is reported for incident energies ranging from 300 to 5000 eV. The present results have been obtained by combining the calculated inelastic electron scattering cross-sections with an experimental energy loss procedure. Calculations have been carried out by means of a quasifree absorption model whose reliability has been checked by comparison with the electron scattering total cross section measured in a transmission beam experiment. Results for O-2 are compared with high energy stopping power data available in the literature. For O-3, these are the first results of the stopping power for electrons. (C) 2003 Elsevier Science B.V. All rights reserved.