The translational energy in O(3s 3S) excited in the dissociative excitation of NO and O 2, by metastable helium (2 3S) has been determined using a resonance absorption technique in the vacuum UV. The absorption data are analyzed using two different emission line profiles: a gaussian profile from a high temperature Maxwell-Boltzmann source, and a trapezoidal profile from a monoenergetic source. The analysis using the gaussian profile indicates an average translational energy of the excited O-atom of ⩾ 0.60 eV from the He* + O 2 source and ⩾0.23 eV from the He* + NO source. The analysis using the monoenergetic profile gives excited O-atom translational energies of ⩾0.30 eV from He* + O 2 and ⩾0.15 eV from He* + NO. There is a marked reduction in effective O-atom translational energy with increased total pressure for both the He* + O 2 and He* + NO cases.