The ONIOM method has been used to model the isolated hydroxyl group at the silica surface using the octahydrosilasesquioxane cluster. Three different bi-layer clusters have been defined, using five levels of theory for the real system. For the model system, the B3-LYP/DZP level has been adopted for all calculations. Geometries, OH vibrational stretching frequency, NMR isotropic delta(H-1) and delta(Si-29) chemical shifts and the binding energies of NH3 computed with the ONIOM approach have been compared with the full ab initio results. Whereas the ONIOM(B3-LYP/ DZP:HF/3-21G) level gives the best results, the cheapest ONIOM(B3-LYP/DZP: AM1) level can be adopted to model very large portions of siliceous materials.