Photoabsorption cross section measurements of the Herzberg I bands (A(3) Sigma(u)(+)-X(3) Sigma(g)(-)) of O-2 have been made by Fourier transform spectrometry with a resolution of 0.06 cm(-1) in the wavelength region 240-270 nm. Precise band oscillator strengths of the (4,0)-(11,0) bands are obtained by direct measurement, and for some of the strong bands; they are significantly higher than the previous experimental values. The rotational line strengths and the branching ratios are also presented for the same bands. The dissociation energy of O-2 is discussed.