Several silicon oxycarbide glasses with O/Si ratios 1.2 and 1.8 were prepared by pyrolysis below 1000 degrees C of organosilicon precursors. Their structure was investigated using Si-29 magic-angle spinning nuclear magnetic resonance. The spectra obtained are consistent with a purely random distribution of Si-O and Si-C bonds, which depends on the O/Si ratio of the glass only, regardless of the structure of the precursors. This result is interpreted by the occurrence, during the pyrolysis of entropically controlled redistribution reactions involving the exchange of Si-O and Si-C bonds.