[en] We have studied the normal modes of hydrogenated and oxidized silicon nanocrystals, namely SiH₄ (silane), H₂SiO (silanone), Si₁₀H₁₆, and Si₁₀H₁₄O. The small clusters (SiH₄ and H₂SiO) have been used for convergence tests and their bond-lengths and frequencies have been compared with experimental and theoretical reference data. For the large clusters (Si₁₀H₁₆ and Si₁₀H₁₄O) we have investigated the vibrational density of states, where we have identified the oxygen-related spectral features. The vibrational modes have also been analyzed with respect to the displacement patterns. The calculations have been carried out within the density-functional and density-functional perturbation theory using the local-density approximation