A strong LR fluorescence is observed following the excitation of O-3 at the nu(1)+nu(3) mode in xenon matrices. The emission frequency is characteristic of the 2 nu(3) to nu(3) transition. The rise time and the decay time are both faster than a few nanoseconds. The excitation spectra are similar to the line profile of the nu(1)+nu(3) absorption at 5 K. A spectral analysis of the fluorescence shows that no energy transfer occurs during the relaxation between molecules in the two trapping sites. The most perturbed molecules (high frequency component of the absorptions) seem to have a faster relaxation, exhibiting a stronger temperature dependence. The possible intramolecular vibrational cascades are discussed considering the literature data about vibrational relaxation of matrix isolated small polyatomic molecules in their ground electronic state.