Optical oscillator strengths for the seven most intense dipole-allowed transitions in the 100-eV incident energy electron scattering spectrum of atomic oxygen have been measured from forward scattering spectra. An apparatus was used in which a microwave discharge source, producing atomic oxygen seeded in a helium beam, was coupled to an electron spectrometer. The O 1D population in the target beam was less than 1.5% of O 3P, and transitions from the O 3P ground electronic state only were observed. The performance of the apparatus was checked by measurement of optical oscillator strengths for helium and atomic hydrogen. Agreement with the National Bureau of Standards (NBS) values for a single spectrum of better than 10% was obtained for three He transitions and 13% for the atomic hydrogen n=3 transitions when the He 11S--21P and H n=1-n=2 Lyman series transitions were normalized to the NBS values. The atomic oxygen 2p4 3P-2p33s 3S¿ (1304 ¿) transition was normalized to an average experimental value of f=0.048. Optical oscillator strengths obtained for the other transitions were 2p4 3P-2p34s 3S¿ (1040 ¿), 0.010¿0.002; 2p4 3P-2p33d 3D¿ (1026 ¿), 0.019¿0.001; 2p4 3P-2p33s 3D¿ (989 ¿), 0.061¿0.006; 2p4 3P-2p34d 3D¿ (972 ¿), 0.016¿0.006; 2p4 3P-2p3s' 3P¿ (878 ¿), 0.086¿0.006; 2p4 3P-2s2p5 3P¿ (792 ¿), 0.070¿0.004. (All values have ¿5% uncertainty in addition to the experimental scatter as indicated.) The electric dipole-forbidden transition 2p4 3P-2p33p 3P was also observed as a weak process.