The absolute optical oscillator strength density spectrum of oxygen has been measured at an impact energy of 1.5 keV and a mean scattering angle of 0° in an energy region of 5.7—28.5 eV with typical FWHM 60 meV.The absolute optical oscillator strengths corresponding to vibrational levels of E3Σ-ｕ,ν′=0,1,2 and 23Πｕ,ν′=0,1 are reported and compared with previous experimental and theoretical results.The validity and accuracy of the Bethe-Born conversion factor is confirmed down to 6 eV by comparison.