[en] The influence of oxygen on the electronic properties of semiconductors is not well known. This is in part due to the fact that the analysis of this element at low concentrations is difficult; the determination of the lattice location is difficult as well. Using charged particle activation (CPA), one is able to analyse oxygen accurately below the ppm level. By combining CPA with channeling, one can also obtain information on the lattice position (s) occupied by the oxygen atoms. The method is described and discussed. Recent work on Ga_1-X Al_XAS prepared by vapour phase epitaxy with organometallic compounds will be presented as an example. It is shown in this work that oxygen can occupy at least two different sites: a tetrahedral interstitial site and another site, probably an arsenic site. The number of atoms on each site is governed by the number of arsenic vacancies and by the total oxygen concentration, which are both governed by the growth conditions. (author). 7 refs., 3 figs., 1 tab