Organically modified silicates have been prepared, using methyltriethoxysilane and tetraethoxysilane in various molar ratios. The final network is thus composed of trifunctionnal T units and tetrafunctionnal Q units. Three types of oxo bridges can be formed through the competitive self-condensation or co-condensation reactions : (T)-O-(T), (Q)-O-(Q) and (T)-O-(Q). 17O-enriched water was used as reactant in order to selectively enrich the various oxo bridges, and to perform 17O solid state MAS-NMR. The combination of MAS experiments recorded at two different magnetic fields, and of the recently discovered MQ-MAS experiments allowed us to identify clearly the three types of oxo bridges. Quantitative analysis was extracted from the MAS-NMR spectra, and compared with calculations based on randomly distributed Si units.