In order to contribute to the understanding of the origin of biaxial nematic ordering in tetrapodes, a deuterium NMR study was performed on mixtures of monomers from organosiloxane tetrapodes with a deuterated nematic probe. Contrary to the tetrapode system previously studied, which exhibits a biaxial nematic phase, the results for monomers are compatible, within the experimental error, with uniaxial nematic ordering in the whole nematic range. The data are in agreement with the conjecture that the nematic biaxial behaviour is related to hindering of the mesogenic units' rotational movements, arising from interdigitation and connection to the central silicon core.