Using OH adsorbed on the copper(111) surface as model system we demonstrate the feasibility of calculations of NEXAFS spectra of chemisorbed species. We employ a static exchange method that is implemented for large systems through direct atomic orbital techniques. The general cluster convergence of the spectra ranging up to 49 cluster atoms is investigated and found to be satisfactory. The special character of the strong metal to adsorbate charge transfer involving occupied OH orbitals and of the OH-metal ionic bond are shown to have particular consequences for both the analysis and the computation of the NEXAFS spectra. For the larger clusters the discrete spectra are dominated by adsorbate to cluster transitions, which can be understood from the similarity of electronic structure of the adsorbed OH moiety with that of free OH- for which all transitions end up in the continuum.