Optical emission spectroscopy has been established as a valuable method for the analysis of broad oxygen ion beams. The ion beams used for reactive ion‐beam etching have been investigated in the energy range of 300–1500 eV. From survey spectra O+2molecules and neutral O atoms are identified as main emitting species. Concerning the occurrence of emission lines the beam spectrum resembles that obtained from an O2rf plasma. The intensity ratios however are strongly different within both spectra. Whereas electron impact is the main source for electronic excitation in ordinary etch plasmas, heavy particle collisions are suggested to play an important role in the investigated ion beams. Beam‐induced emissions of atomic oxygen neutrals were recorded at high resolution of 0.1 A˚. The O emission lines were found to be triple peaked. One peak at the unshifted wavelength and two Doppler‐shifted peaks could be resolved. The absolute values of the wavelength shifts are well correlated to the energies of the initial ions extracted from the ion source by a grid optics. Besides slow atoms, atoms moving with the full beam energy as defined by the ion extraction conditions and with only half the beam energy are detected. The observed Doppler structure is attributed to charge exchange and dissociative collisions taking place in the gas phase.