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The electron energy loss spectra for the valence-shell excitations of oxygen are measured at an incident electron energy of 2500 eV and at scattering angles from 4°to 8.5°.The dipole-forbidden and octupole-allowed transition of the A3△u←X3Σ-g is observed from the measured spectra,and it is found that the peak profile of the A3△u←X3Σ-g excitation can be well represented by a Gaussin function.The energy position of 6.007±0.008 eV and Franck-Condon linewidth of 1.312±0.020 eV of the A△u←X3Σ-g,which are independent of the scattering condition,have been determined for the first time.The peak profiles of the A3△u←X3Σ-g determined in this work provide a possibility to realize the spectral decomposition method of unfolding the Herzberg pseudocontinuum proposed by Campbell et al.[Phys.Rev.A 61 (2000) 022706]